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| DETERMINING CASE LENGTHS, CHAMBER CASTS & IMPACT IMPRESSIONS By Wayne McLerran |
Table of Contents:
Introduction
The Quick Method
Trim-&-Try Method
A Few Words on Sulfur Chamber Casts
Cerrosafe Chamber Casts
Comments & Cautions
Making a Cerrosafe Cast
Adjusting Data from “Aged” Casts
Advantages & Disadvantages
Chamber Mouth & Throat Impact (Swaged) Impression
Making an Impact Impression
Advantages & Disadvantages
Introduction
Prior to jumping into the following lengthy discussion, I’d like to make it
clear that my comments on measuring case lengths are limited to black
powder cartridge rifle (BPCR) straight wall cases. Determining the
correct length of BPCR bottle-neck cases requires the same techniques
used with smokeless bottle-neck cases, and will not be discussed here.
The steps involved in making chamber casts or impact impressions
apply to both BPC and smokeless firearms.
Firing a BPCR with a case that’s either too long or too short can have a
direct affect on accuracy and lead to other problems. A case that’s too
long can result in increased chamber pressures and will affect accuracy
due to bullet deformation and base fining. A case that’s too short is the
most common situation. One result of a short case is powder and/or
lead fouling accumulating in the gap between the case lip and the
transition step. Depending on the width of the gap and the position of
the bullet driving bands and lube rings in relation to the gap, lube may
completely fill the gap, or the bullet may expand to fill the gap with lead.
Assuming the gap is filled with lead, the wider the gap the more the
expanded bullet will be swaged down by the transition step and throat,
resulting in lead smeared back to the base edge, forming an irregular
cupped or “finned” base, which is certain to degrade accuracy. Under
these circumstances it’s also common to find a lead ring in the front
portion of the chamber when lead is stripped off by the transition step.
For an additional discussion on shooting short-case cartridges go to
SHOOTING SHORT-CASE CARTRIDGES IN BPCRs - MYTHS &
FACTS. As you read further, keep in mind that one of the key goals of
every knowledgeable rifle designer, reloader and shooter is minimizing
bullet deformation.
The location of the transition step is the key element in determining the
correct case length. Therefore a brief discussion on the transition step
is warranted. It’s the very short and sharply angled region where the
chamber narrows down to the smaller groove and bore diameters of the
throat. The typical transition step has a 45 degree angle, but the angle
can be much less, especially for chambers designed to handle cast
bullets, and is a necessary part of accurate chamber and bore designs of
cartridge firing rifles. It allows the chamber to accept the larger diameter
of the cartridge case and fire a bullet with a smaller diameter to match the
bore while minimizing bullet deformation. The transition step is crucial in
rifles firing jacketed or hard bullets. It’s less critical in BPC rifles since the
softer bullets will generally expand to fill the bore, but in doing so the
bullet will be deformed to some extent.
The case length is ideal when the front edge of the case mouth just
barely contacts the transition step when the cartridge is fired. The ideal
overall length (OAL) of a fire-formed case (fired but not resized)
positions the front edge of the case mouth so that it almost comes into
contact with the rear edge of the transition. My definition of “almost”
leaves some space (not much) for case stretching under firing conditions.
The case may stretch some and then retract when fired. Therefore, if the
front edge of the case mouth is in hard contact with the transition step,
“fire crimping” will result, essentially rolling over the case edge and
“squeezing” the bullet to a smaller diameter as it exits the case. Although
the relatively soft cast bullet will expand (obturate) to fill the throat and
bore, increased fining of the base edge caused by fire crimping the case
mouth may have a negative affect on accuracy. Once you’ve determined
the absolute maximum case length (mouth edge in hard contact with the
transition step) of a fire-formed case, I recommend shortening the case
0.005” or so to eliminate the possibility of fire crimping. To determine,
with a higher degree of accuracy, how much your cases initially stretch, if
any, prior to shrinking back when fired will require further experimentation
on your part.
So how does one determine the absolute maximum case length? There
are five methods that come to mind. There’s the Quick Method and one
I call the Trim-&-Try Method. The third and forth method consists of
making a chamber cast using either sulfur or a lead alloy. The final
method discusses making an impact impression of the chamber and
is the technique I prefer to use and highly recommend over the others.
All will work, but there are benefits and trade offs associated with each
technique.
The Quick Method
The quick method requires a case longer than the chamber; an example
is using a .45-90 case to measure a .45-70 chamber. The neck of the
case must be annealed to a very soft condition. I stand the case up in
a tuna can almost filled with water. The case neck is heated with a
common propane torch until it starts to glow red and then quickly
knocked over in the water to quench it. Now lube the outside of the case
and insert it into the chamber. Using a wooden dowel and hammer or
similar tools, drive the case in until the rim fully seats. It should go in
easily. Extract the case and it should be obvious where it was reduced
in diameter by the transition step. Measure the length to the start of the
transition step and you have the overall case length for your chamber.
One drawback to this method is it can be a little “tricky” to determine the
exact start of the transition step, but your measurement should be close
enough. The lands and grooves should show as shallow impression on
the end of the case, but I do not recommend measuring them to
determine bore dimensions. Slugging the bore will provide good and
accurate measurements of the groove and bore diameters.
Trim-&-Try Method
In my opinion the trim-&-try method is the most problematic way to
determine the correct case length. The initial difficulty is in obtaining an
experimental case with the correct diameter and slightly longer than the
ideal overall case length. Someone that’s reforming down larger caliber
cases to use in a smaller caliber rifle will find the process a little easier.
The reformed untrimmed case will generally end up longer than required,
a good thing. One example is reforming .45-70 Gov. cases to fit into a
.40-65 Win. chamber. Assuming availability of a sufficiently long case, it
must also be of the proper diameter.
In order for the mouth of the extended case to properly contact and stop
at the transition step it must be sized or neck expanded so that it just
barely slides in. Slightly flaring the lip can help. If the case mouth outside
diameter (OD) is very close to the chamber OD, the case mouth will stop
when it comes into contact with the start of the transition step, indicating
the front end of the chamber. If the case diameter is too small the lip
may slide past the transition step resulting in a case that’s too long.
Once the properly dimensioned but too long case contacts the transition
step, start trimming the case in small increments until it just barely but
fully seats into the chamber with the action closed, hence the term
trim-&-try. You now have the correct length of a fire-formed unsized
case. In addition to the problems associated with this method, it will not
provide dimensional data of the rifle throat, an added benefit when using
one of the following techniques.
A Few Words on Sulfur Chamber Casts
Earlier I mentioned using sulfur to make a chamber cast. Prior to the
development of Cerrosafe as the preferred chamber cast material, using
a melted mixture containing mostly sulfur, commonly know as the Baker
mixture, was the time-honored technique. Sulfur does offer a couple of
benefits: it’s cheap and readily available from local home and garden
supply stores or pharmacies. Pharmaceutical sulfur comes in three
forms: “flowers of sulfur” is made by sublimation; the other two forms
are precipitated sulfur and washed sulfur, also known as sublimed sulfur.
One negative in using sulfur is it’s somewhat dangerous to melt and can
ignite with an almost invisible flame. It's also messy and the casts are
very brittle. Since I have not used sulfur, I will not discuss the details of
making sulfur casts. If you are interested, go to the following link http:
//longrangehunting.com/forums/f19/chamber-cast-3734/ or search the
Internet. Roy F. Dunlap also covers the technique in his very well-know
book titled Gunsmithing, which was first published in 1950 and followed
by a 2nd edition in 1963.
Cerrosafe Chamber Casts
Comments & Cautions
Prior to discussing the steps involved in making a Cerrosafe chamber
cast I thought I’d briefly cover some background information on the
alloy and the company that produces it. Cerrosafe is one of many
casting alloys produced by Bolton Technologies. Bolton was formerly
known as Cerro Metal Products Company (hence the name Cerrosafe).
The name changed in February 2007 when the company was acquired
by the Bolton Group. Bolton Technology is headquartered in Bellefonte,
Pennsylvania.
Cerrosafe is one of a family of “Cerro” casting alloys and is comprised
of bismuth (42.5%), lead (37.7%), tin (11.3%) and cadmium (8.5%).
Bismuth is the essential ingredient, the percentage of which determines
if the alloy will shrink or expand after a cast is made. Cerrosafe has a
melting range of approximately 160 to 190 degrees F. It’s available
from several gunsmith supply companies and many firearms accessory
retailers in ½ lb or larger ingots. Three suppliers I’m aware of are
Brownells, MidwayUSA and Buffalo Arms. No doubt there are others.
The typical Cerrosafe cast of a .45-70 chamber, including the throat,
weighs approximately 3.4 oz; a .45-90 cast a little more, a .40-65 cast a
little less. Therefore a ½ lb ingot should be sufficient to make two casts.
A few words of caution are warranted here concerning handling
Cerrosafe. We are all aware of the harmful affects of inhaling or
digesting lead. I understand cadmium is even worse. Although the
percentage of cadmium in Cerrosafe is relatively small, it’s considered
toxic even in low concentrations and will bioaccumulate in organisms
and ecosystems. So use extra caution and be sure to wash your hands
after melting and handling Cerrosafe. The lungs absorb cadmium more
efficiently than the stomach, so it’s important not to exceed the melting
temperature range as dangerous fumes may be generated.
If all you need is one or two casts and have no plans to measure the
cast dimensions again, ignore the comments on Cerrosafe’s expansion
rates except for the important 1hr measurement point. But if you plan
on keeping the cast for future measurements, than a clear understanding
of the expansion characteristics are necessary to insure the data
accurately represents the true dimensions of the chamber and throat
of your rifle. An in-depth discussion on expansion characteristics
follows the details on making a cast.
Making a Cerrosafe Cast
It’s well known that once the cast is poured Cerrosafe shrinks and then
starts to expand over time as it “ages”. Not specifically mentioned in
some instructions is the fact that Cerrosafe will expand to match the
chamber dimensions approximately 1 hr after the cast is made, the
ideal time to take measurements. Measurements taken before or after
1 hr will require a mathematical adjustment. You don’t have to rush, a
few minutes one way or the other won’t make a significant difference,
but 10 or 15 minutes will. So have a vernier caliper or micrometer handy
and determine what data you need prior to the measurement time. If
you forget and miss the measurement time you’ll have to make
adjustments to the data (see later instructions) or remake the cast.
Although disassembling the rifle action is not mandatory and may not be
recommended for some rifles, I find it simplifies and helps when making
a chamber cast for two main reasons: it allows sufficient access to pour
the alloy in the chamber without the need of a pouring tube and also
eliminates or at least reduces the possibility of spillage into parts of the
action.
The following steps are a compilation of instructions and comments
drawn from my technique and from several other sources:
• Remove the barreled action from the stock and forearm and
disassemble the rifle as needed to gain access to the chamber.
• A rifle that does not offer good chamber access may require a pouring
tube and funnel. The tube and funnel can be made from brass, copper,
aluminum or steel tubing and should be as short as possible to minimize
Cerrosafe solidifying in the tube. Use an existing metal funnel or make a
temporary one out of thin sheet metal, brass shim stock or a few layers
of aluminum foil (the easiest method).
• Clean and lightly oil the chamber and bore.
• Fold up a large clean cotton cleaning patch and roll or form it into a
cylinder shape until it will tightly fit into the bore from the muzzle. Using
a cleaning rod, force the patch down the bore until it reaches a point
approximately 1/2” to 1” in front of the mouth or throat of the chamber.
This will serve as a “dam” for the Cerrosafe.
• Secure the rifle vertically (chamber end up) in a padded vice or similar
holder while taking precautions to protect the rifle from damage.
• Heat the barrel around the chamber with a propane torch or hot air gun
to a point that it’s good and warm but still comfortable to hold by hand.
I'd say 100 degrees is about right if that gives you a better idea. Do not
overheat.
• Melt the Cerrosafe in a double boiler or use other sources of indirect
heat. Caution - applying direct heat and/or overheating may lead to
separation of the components of the alloy or lead to the release of
dangerous fumes. A large clean bullet casting ladle or plumbers ladle
can be used to heat and pour the alloy, using a propane torch to heat
the ladle from the bottom. I prefer to place the Cerrosafe in a bent tuna
can and use a hot air gun for heating. The tuna can is bent to form a
convenient pouring spout and placed on a brick for heating.
• “Vice-Grip"-type locking pliers can be used as a handle for the can.
Since the melted Cerrosafe should not be more than 190 degrees F
(less than the temperature of boiling water), I prefer to use heavy gloves
such as welding gloves to handle the tuna can. It provides for better
control while pouring.
• As quickly and as safely possible, fill the chamber until it forms a slight
mound at the breech or rear of the chamber. Over pouring will result in
excess alloy spilling over the breech and likely prevent removal of the
cast, especially if the action has not been disassembled. If you’re using
a pouring tube it may be necessary to have a helper heat the tube with a
propane torch while pouring.
• Wait for the Cerrosafe to solidify to push it out of the chamber. It’s a
good idea to wait approximately 20 to 30 minutes for maximum
shrinkage. Any longer and the chances increase that the cast will not
come out as it starts to enlarge
• Flip the rifle over (muzzle pointing up) and secure it once again in the
vice.
• While waiting to remove the cast fold up a soft rag or towel and place
it on the floor below the inverted rifle. It will gently catch the cast as it’s
pushed out of the chamber.
• Remove the cast using a cleaning rod inserted from the muzzle and
tapped with the heel of your hand. If the cast will not budge and
something heavier would be required to drive the cast out than you’ve
probably waited too long. In that case, heat the barrel with a propane
torch or hot air gun sufficiently to melt the alloy. Allow it to cool for
½ hr and try tapping it out again. If it will not budge on the 2nd attempt
you’ll have to melt the alloy again and pour it out of the barrel.
• With the cast in hand, check out the surface condition. Many wrinkles
or large surface voids are an indication that either the alloy was not hot
enough or the barrel was not sufficiently warm. Heavy frosting on the
surface is an indication of overheating of the alloy or barrel. If either of
these conditions prevents a good measurement then make a new cast.
So, now you’ve successfully made a Cerrosafe cast. Having taking
careful measurements after waiting the recommended 1hr you’ve
decided to keep the cast for future measurements. In that case, be
sure to document the date and time the cast was made. I apply Scotch
tape around the middle of the cast and write the info on it with a
permanent ink felt tip pin. I’ve inadvertently mixed up casts of the same
caliber and find this to be a better method than labeling a storage
envelope. Better yet, do both.
Adjusting Data from “Aged” Casts
The following information on expansion characteristics is from my
research and experiences in making well over 50 casts to perfect my
casting technique and gather chamber and bore information for my book
on the Browning BPCRs. I still have the original casts which were made
several years ago and have taken additional data months and years later
that directly correlates back to the original cast dimensions.
I mentioned Buffalo Arms and Brownells as two suppliers of Cerrosafe.
The cast instructions provided by Buffalo Arms does not discuss
expansion characteristics, but Brownells’ data sheet does discuss it.
Based on my experiences, I recommend that you completely ignore the
expansion comments provided by Brownells for two reasons. It became
clear to me that Brownells’ data did not correlate to the actual data I
measured over time. Plus, after discussing the discrepancies with a
Brownells technical representative, he could not substantiate the data
and only confirmed that the measurements should be taken 1hr after the
cast was made, essentially indicating that their expansion data may be
incorrect. Since then I have definitely confirm it's not correct.
Although I have not purchased Cerrosafe from MidwayUSA, early on,
as I was becoming familiar with the process and found that Brownells’
expansion correction factors were not correct, I shared my frustrations
and findings with a shooter very experienced with using Cerrosafe. He
referred me to MidwayUSA’s instructions and data sheet on the product,
which contained the accurate expansion factors listed below.
Contraction & expansion factors versus time, measured in inches per
square inch: (Note that the factors are referenced to the suggested 1 hr
measurement point)
• 2 minutes -.0004"
• 6 minutes -.0007"
• 30 minutes -.0009"
• 1 hour .0000"
• 2 hours +.0016"
• 5 hours +.0018"
• 7 hours +.0019"
• 10 hours +.0019"
• 24 hours +.0022"
• 96 hours +.0025"
• 200 hours +.0025"
• 500 hours +.0025”
So, other than to confirm that Cerrosafe does in fact initially contract
then expand as it cools and ages over time, how useful are the actual
factors. To start with, notice that the maximum shrinkage listed is at
30 minutes, which is the ideal time to remove the cast from the
chamber. Also note that the sign of the factor changes from negative
to positive 1 hr after the cast is made (the point in time when the cast
dimensions match the chamber dimensions). It may not be clear to
you now, but the sign is important when the factor is used in a simple
formula to adjust data taken from aged casts. Also note that at 96
hours (4 days), Cerrosafe has reached its maximum expansion point
and stabilized; years later it will measure the same as it did at the 96 hr
point.
Another key to using the data is noted in the table heading indicating
that the contraction & expansion factors versus time are measured in
inches per square inch. So now you’re probably scratching your head
and thinking, what in the devil does that mean? It certainly wasn’t clear
to me at first. What it actually means is, in order to use the correction
factor, it must be subtracted from the square of the measurement,
then take the square root of the result to obtain the corrected data.
For example, the cast neck diameter from my Browning .40-65 BPCR
chamber measured 0.4353” one hour after taking the cast. 24 hrs later
the cast neck measured 0.4375”. So, 0.4375” squared equals
0.19141”. Next, the appropriate factor (0.0022”) is subtracted from
0.19141”, which equals 0.18921”. Now, taking the square root of
0.18921” equals 0.4350”; a difference of only 0.0003” from the original
1 hr measurement. Now that’s real close.
Another example, the cast groove diameter from my Browning .45-70
BPCR measured 0.4590” one hour after taking the cast. Well over 96
hrs (actually 4 years) later the groove diameter from the cast measured
0.4615”. So, 0.4615” squared – 0.0025” = 0.2105”. Now, taking the
square root of 0.2105” = 0.4588”; only 0.0002” difference from the 1 hr
measurement, which is better than my measuring ability.
By the way, you might be thinking to yourself that the factors and math
may work fine to convert aged measurements to the 1 hr data, but how
can you be sure the 1 hr measurements accurately reflect the chamber
and bore dimensions? I had the same concern and verified several
chamber and bore dimensions using soft lead slugs. The Cerrosafe
measurements, taken at approximately 1 hr after the casts were made,
were “right on” within my measuring ability. I also used impact
impressions (discussed below) to measure chamber lengths and
compared the data to measurements taken from Cerrosafe casts.
The data agreed in all cases.
Advantages & Disadvantages
So what are the advantages and disadvantages to using Cerrosafe?
One advantage is the relative hardness of the cast. It’s not as easily
damaged when taking measurements as is an impact impression. If the
cast is sufficiently long to include a short section of the lands, it will
provide dimensional data of the throat and bore, but the same benefit
applies to the impact impression. Also, a Cerrosafe cast provides
diameter data along the entire chamber, including the center and rear
portion of the chamber; data which is not available from an impact
impression. For most shooters this data is not particularly important so
I don’t consider this much of an advantage, if any.
Disadvantages include: handling concerns & hazards, dealing with
expansion characteristics, having to partially or fully disassembling the
action to pour and remove the cast, and not being able to directly
measure case overall length (OAL). The latter two are the main
reasons I prefer an impact impression.
Chamber Mouth & Throat Impact (Swaged) Impression
In lieu of making a chamber cast with sulfur or Cerrosafe, I prefer and
highly recommend the following technique, which is also commonly
referred to as making an impact casting or impact slug.
Making an Impact Impression
Starting with a fire-formed case, which has not been resized, shorten the
length by approximately 0.200” (the amount is not critical) and fill it with
lead to approximately ½” below the mouth.
You’ll need a sufficient amount of pure or nearly pure lead slightly
smaller than the bore diameter. It will be dropped down the bore and
compressed to form the impression. If bullet lead is not available, soft
lead wire can be found in the fishing section of most sporting goods
stores.
Clean, lightly oil the chamber and insert the partially lead-filled fire-formed
case.
Next you’ll need a steel rod just under bore (land) diameter, available at
most hardware stores. The rod should be a few inches longer than the
barrel with a squared-off cleanly finished end. Wrap the squared-off end
with tape to make a snug fit to the bore, which will help in centering the
end of the rod and eliminate any possibility of the edge galling against
the bore. If a rod of the proper diameter is used, the possibility of
damaging the rifling lands is just about nonexistent. Taping the end that
contacts the impression is just an extra precaution in case the squared
off end is irregular or not properly finished. The only other location I'd
consider taping is the section that comes in contact with the muzzle to
protect the crown.
With the rifle standing up, butt stock on the floor; drop a sufficient
amount of lead down the muzzle to fill the chamber and throat when the
impression is completely formed. Be conservative and don’t use too
much lead or the impression will be too long, extending down the bore
and hard or impossible to remove without damaging it. If, after
removing the impression, you find that the amount of lead was not
sufficient, it’s easy to redo with the addition of more lead.
Using a piece of paper towel, insert a wad of around 1/2" into the
muzzle end. The paper will compress, fill the grooves and prevent the
rod from embedding itself and seizing on the end of the compressed
lead impression.
Insert the wrapped end of the rod into the muzzle until it contacts the
paper wad. Now strike the rod with measured even stokes, just enough
to fully compress the soft lead. When the lead is fully compressed you'll
feel the difference as the rod will start bouncing with every stroke of
the hammer.
Open the action and gently push the case and lead impression out with
the steel rod. Fold up a soft cotton rag or towel to catch the impression
as it’s pushed out of the chamber. Be gentle so as not to further deform
the lead impression. If the correct amount of lead was used, the
impression should neatly form to the dimensions of the neck and mouth
of the chamber, the throat and a small section of the bore. If it’s not
long enough, repeat the process after adding more lead.
Note - When adding additional lead to an existing impression, I've had
good success with roughing up the end of the lead impression,
reinserting it, adding additional lead down the bore and swaging a new
impression. In all but a couple of cases the new lead firmly adhered to
the old impression and the seam was virtually invisible. The couple of
times the added lead did not stick well were most likely due to oil
contamination. If, after roughing up the end of the old impression, you
still have a problem with the added lead adhering, essentially start over
by cutting off the lead extending out of the case and drill a little out of the
mouth to a depth of 0.100” to 0.200”, or heat up the lead-filled case and
pour a little out, which may be the easier way.
Advantages & Disadvantages
There are several advantages using this technique. It’s cheaper, simpler
and easier than using Cerrosafe. The rifle action does not require
disassembly and there’s no chance of spilling excess sulfur or Cerrosafe
into portions of the action. Measuring the impact impression from the
rear of the rim to the start of the transition step includes the actual
headspace clearance of the rifle and will provide the correct OAL of a
properly trimmed fire-formed case. Plus, if the impression is sufficiently
long to include a short section of the lands, it will provide dimensional
data of the throat and bore. Finally, there’s no concern about the
impression changing dimensions over time.
There are a couple of disadvantages which I feel are not significant, but
should be discussed. Being made from soft lead, one negative is the
ease of damaging the impression or slug if it’s not handled with
reasonable care. Of course, if damaged, it’s relatively simple to make
another one. Another disadvantage is the inability to accurately measure
diameters along the center portion and rear of the chamber; data that's
generally not required or useful.
Wishing you great shooting,
Wayne
Introduction
The Quick Method
Trim-&-Try Method
A Few Words on Sulfur Chamber Casts
Cerrosafe Chamber Casts
Comments & Cautions
Making a Cerrosafe Cast
Adjusting Data from “Aged” Casts
Advantages & Disadvantages
Chamber Mouth & Throat Impact (Swaged) Impression
Making an Impact Impression
Advantages & Disadvantages
Introduction
Prior to jumping into the following lengthy discussion, I’d like to make it
clear that my comments on measuring case lengths are limited to black
powder cartridge rifle (BPCR) straight wall cases. Determining the
correct length of BPCR bottle-neck cases requires the same techniques
used with smokeless bottle-neck cases, and will not be discussed here.
The steps involved in making chamber casts or impact impressions
apply to both BPC and smokeless firearms.
Firing a BPCR with a case that’s either too long or too short can have a
direct affect on accuracy and lead to other problems. A case that’s too
long can result in increased chamber pressures and will affect accuracy
due to bullet deformation and base fining. A case that’s too short is the
most common situation. One result of a short case is powder and/or
lead fouling accumulating in the gap between the case lip and the
transition step. Depending on the width of the gap and the position of
the bullet driving bands and lube rings in relation to the gap, lube may
completely fill the gap, or the bullet may expand to fill the gap with lead.
Assuming the gap is filled with lead, the wider the gap the more the
expanded bullet will be swaged down by the transition step and throat,
resulting in lead smeared back to the base edge, forming an irregular
cupped or “finned” base, which is certain to degrade accuracy. Under
these circumstances it’s also common to find a lead ring in the front
portion of the chamber when lead is stripped off by the transition step.
For an additional discussion on shooting short-case cartridges go to
SHOOTING SHORT-CASE CARTRIDGES IN BPCRs - MYTHS &
FACTS. As you read further, keep in mind that one of the key goals of
every knowledgeable rifle designer, reloader and shooter is minimizing
bullet deformation.
The location of the transition step is the key element in determining the
correct case length. Therefore a brief discussion on the transition step
is warranted. It’s the very short and sharply angled region where the
chamber narrows down to the smaller groove and bore diameters of the
throat. The typical transition step has a 45 degree angle, but the angle
can be much less, especially for chambers designed to handle cast
bullets, and is a necessary part of accurate chamber and bore designs of
cartridge firing rifles. It allows the chamber to accept the larger diameter
of the cartridge case and fire a bullet with a smaller diameter to match the
bore while minimizing bullet deformation. The transition step is crucial in
rifles firing jacketed or hard bullets. It’s less critical in BPC rifles since the
softer bullets will generally expand to fill the bore, but in doing so the
bullet will be deformed to some extent.
The case length is ideal when the front edge of the case mouth just
barely contacts the transition step when the cartridge is fired. The ideal
overall length (OAL) of a fire-formed case (fired but not resized)
positions the front edge of the case mouth so that it almost comes into
contact with the rear edge of the transition. My definition of “almost”
leaves some space (not much) for case stretching under firing conditions.
The case may stretch some and then retract when fired. Therefore, if the
front edge of the case mouth is in hard contact with the transition step,
“fire crimping” will result, essentially rolling over the case edge and
“squeezing” the bullet to a smaller diameter as it exits the case. Although
the relatively soft cast bullet will expand (obturate) to fill the throat and
bore, increased fining of the base edge caused by fire crimping the case
mouth may have a negative affect on accuracy. Once you’ve determined
the absolute maximum case length (mouth edge in hard contact with the
transition step) of a fire-formed case, I recommend shortening the case
0.005” or so to eliminate the possibility of fire crimping. To determine,
with a higher degree of accuracy, how much your cases initially stretch, if
any, prior to shrinking back when fired will require further experimentation
on your part.
So how does one determine the absolute maximum case length? There
are five methods that come to mind. There’s the Quick Method and one
I call the Trim-&-Try Method. The third and forth method consists of
making a chamber cast using either sulfur or a lead alloy. The final
method discusses making an impact impression of the chamber and
is the technique I prefer to use and highly recommend over the others.
All will work, but there are benefits and trade offs associated with each
technique.
The Quick Method
The quick method requires a case longer than the chamber; an example
is using a .45-90 case to measure a .45-70 chamber. The neck of the
case must be annealed to a very soft condition. I stand the case up in
a tuna can almost filled with water. The case neck is heated with a
common propane torch until it starts to glow red and then quickly
knocked over in the water to quench it. Now lube the outside of the case
and insert it into the chamber. Using a wooden dowel and hammer or
similar tools, drive the case in until the rim fully seats. It should go in
easily. Extract the case and it should be obvious where it was reduced
in diameter by the transition step. Measure the length to the start of the
transition step and you have the overall case length for your chamber.
One drawback to this method is it can be a little “tricky” to determine the
exact start of the transition step, but your measurement should be close
enough. The lands and grooves should show as shallow impression on
the end of the case, but I do not recommend measuring them to
determine bore dimensions. Slugging the bore will provide good and
accurate measurements of the groove and bore diameters.
Trim-&-Try Method
In my opinion the trim-&-try method is the most problematic way to
determine the correct case length. The initial difficulty is in obtaining an
experimental case with the correct diameter and slightly longer than the
ideal overall case length. Someone that’s reforming down larger caliber
cases to use in a smaller caliber rifle will find the process a little easier.
The reformed untrimmed case will generally end up longer than required,
a good thing. One example is reforming .45-70 Gov. cases to fit into a
.40-65 Win. chamber. Assuming availability of a sufficiently long case, it
must also be of the proper diameter.
In order for the mouth of the extended case to properly contact and stop
at the transition step it must be sized or neck expanded so that it just
barely slides in. Slightly flaring the lip can help. If the case mouth outside
diameter (OD) is very close to the chamber OD, the case mouth will stop
when it comes into contact with the start of the transition step, indicating
the front end of the chamber. If the case diameter is too small the lip
may slide past the transition step resulting in a case that’s too long.
Once the properly dimensioned but too long case contacts the transition
step, start trimming the case in small increments until it just barely but
fully seats into the chamber with the action closed, hence the term
trim-&-try. You now have the correct length of a fire-formed unsized
case. In addition to the problems associated with this method, it will not
provide dimensional data of the rifle throat, an added benefit when using
one of the following techniques.
A Few Words on Sulfur Chamber Casts
Earlier I mentioned using sulfur to make a chamber cast. Prior to the
development of Cerrosafe as the preferred chamber cast material, using
a melted mixture containing mostly sulfur, commonly know as the Baker
mixture, was the time-honored technique. Sulfur does offer a couple of
benefits: it’s cheap and readily available from local home and garden
supply stores or pharmacies. Pharmaceutical sulfur comes in three
forms: “flowers of sulfur” is made by sublimation; the other two forms
are precipitated sulfur and washed sulfur, also known as sublimed sulfur.
One negative in using sulfur is it’s somewhat dangerous to melt and can
ignite with an almost invisible flame. It's also messy and the casts are
very brittle. Since I have not used sulfur, I will not discuss the details of
making sulfur casts. If you are interested, go to the following link http:
//longrangehunting.com/forums/f19/chamber-cast-3734/ or search the
Internet. Roy F. Dunlap also covers the technique in his very well-know
book titled Gunsmithing, which was first published in 1950 and followed
by a 2nd edition in 1963.
Cerrosafe Chamber Casts
Comments & Cautions
Prior to discussing the steps involved in making a Cerrosafe chamber
cast I thought I’d briefly cover some background information on the
alloy and the company that produces it. Cerrosafe is one of many
casting alloys produced by Bolton Technologies. Bolton was formerly
known as Cerro Metal Products Company (hence the name Cerrosafe).
The name changed in February 2007 when the company was acquired
by the Bolton Group. Bolton Technology is headquartered in Bellefonte,
Pennsylvania.
Cerrosafe is one of a family of “Cerro” casting alloys and is comprised
of bismuth (42.5%), lead (37.7%), tin (11.3%) and cadmium (8.5%).
Bismuth is the essential ingredient, the percentage of which determines
if the alloy will shrink or expand after a cast is made. Cerrosafe has a
melting range of approximately 160 to 190 degrees F. It’s available
from several gunsmith supply companies and many firearms accessory
retailers in ½ lb or larger ingots. Three suppliers I’m aware of are
Brownells, MidwayUSA and Buffalo Arms. No doubt there are others.
The typical Cerrosafe cast of a .45-70 chamber, including the throat,
weighs approximately 3.4 oz; a .45-90 cast a little more, a .40-65 cast a
little less. Therefore a ½ lb ingot should be sufficient to make two casts.
A few words of caution are warranted here concerning handling
Cerrosafe. We are all aware of the harmful affects of inhaling or
digesting lead. I understand cadmium is even worse. Although the
percentage of cadmium in Cerrosafe is relatively small, it’s considered
toxic even in low concentrations and will bioaccumulate in organisms
and ecosystems. So use extra caution and be sure to wash your hands
after melting and handling Cerrosafe. The lungs absorb cadmium more
efficiently than the stomach, so it’s important not to exceed the melting
temperature range as dangerous fumes may be generated.
If all you need is one or two casts and have no plans to measure the
cast dimensions again, ignore the comments on Cerrosafe’s expansion
rates except for the important 1hr measurement point. But if you plan
on keeping the cast for future measurements, than a clear understanding
of the expansion characteristics are necessary to insure the data
accurately represents the true dimensions of the chamber and throat
of your rifle. An in-depth discussion on expansion characteristics
follows the details on making a cast.
Making a Cerrosafe Cast
It’s well known that once the cast is poured Cerrosafe shrinks and then
starts to expand over time as it “ages”. Not specifically mentioned in
some instructions is the fact that Cerrosafe will expand to match the
chamber dimensions approximately 1 hr after the cast is made, the
ideal time to take measurements. Measurements taken before or after
1 hr will require a mathematical adjustment. You don’t have to rush, a
few minutes one way or the other won’t make a significant difference,
but 10 or 15 minutes will. So have a vernier caliper or micrometer handy
and determine what data you need prior to the measurement time. If
you forget and miss the measurement time you’ll have to make
adjustments to the data (see later instructions) or remake the cast.
Although disassembling the rifle action is not mandatory and may not be
recommended for some rifles, I find it simplifies and helps when making
a chamber cast for two main reasons: it allows sufficient access to pour
the alloy in the chamber without the need of a pouring tube and also
eliminates or at least reduces the possibility of spillage into parts of the
action.
The following steps are a compilation of instructions and comments
drawn from my technique and from several other sources:
• Remove the barreled action from the stock and forearm and
disassemble the rifle as needed to gain access to the chamber.
• A rifle that does not offer good chamber access may require a pouring
tube and funnel. The tube and funnel can be made from brass, copper,
aluminum or steel tubing and should be as short as possible to minimize
Cerrosafe solidifying in the tube. Use an existing metal funnel or make a
temporary one out of thin sheet metal, brass shim stock or a few layers
of aluminum foil (the easiest method).
• Clean and lightly oil the chamber and bore.
• Fold up a large clean cotton cleaning patch and roll or form it into a
cylinder shape until it will tightly fit into the bore from the muzzle. Using
a cleaning rod, force the patch down the bore until it reaches a point
approximately 1/2” to 1” in front of the mouth or throat of the chamber.
This will serve as a “dam” for the Cerrosafe.
• Secure the rifle vertically (chamber end up) in a padded vice or similar
holder while taking precautions to protect the rifle from damage.
• Heat the barrel around the chamber with a propane torch or hot air gun
to a point that it’s good and warm but still comfortable to hold by hand.
I'd say 100 degrees is about right if that gives you a better idea. Do not
overheat.
• Melt the Cerrosafe in a double boiler or use other sources of indirect
heat. Caution - applying direct heat and/or overheating may lead to
separation of the components of the alloy or lead to the release of
dangerous fumes. A large clean bullet casting ladle or plumbers ladle
can be used to heat and pour the alloy, using a propane torch to heat
the ladle from the bottom. I prefer to place the Cerrosafe in a bent tuna
can and use a hot air gun for heating. The tuna can is bent to form a
convenient pouring spout and placed on a brick for heating.
• “Vice-Grip"-type locking pliers can be used as a handle for the can.
Since the melted Cerrosafe should not be more than 190 degrees F
(less than the temperature of boiling water), I prefer to use heavy gloves
such as welding gloves to handle the tuna can. It provides for better
control while pouring.
• As quickly and as safely possible, fill the chamber until it forms a slight
mound at the breech or rear of the chamber. Over pouring will result in
excess alloy spilling over the breech and likely prevent removal of the
cast, especially if the action has not been disassembled. If you’re using
a pouring tube it may be necessary to have a helper heat the tube with a
propane torch while pouring.
• Wait for the Cerrosafe to solidify to push it out of the chamber. It’s a
good idea to wait approximately 20 to 30 minutes for maximum
shrinkage. Any longer and the chances increase that the cast will not
come out as it starts to enlarge
• Flip the rifle over (muzzle pointing up) and secure it once again in the
vice.
• While waiting to remove the cast fold up a soft rag or towel and place
it on the floor below the inverted rifle. It will gently catch the cast as it’s
pushed out of the chamber.
• Remove the cast using a cleaning rod inserted from the muzzle and
tapped with the heel of your hand. If the cast will not budge and
something heavier would be required to drive the cast out than you’ve
probably waited too long. In that case, heat the barrel with a propane
torch or hot air gun sufficiently to melt the alloy. Allow it to cool for
½ hr and try tapping it out again. If it will not budge on the 2nd attempt
you’ll have to melt the alloy again and pour it out of the barrel.
• With the cast in hand, check out the surface condition. Many wrinkles
or large surface voids are an indication that either the alloy was not hot
enough or the barrel was not sufficiently warm. Heavy frosting on the
surface is an indication of overheating of the alloy or barrel. If either of
these conditions prevents a good measurement then make a new cast.
So, now you’ve successfully made a Cerrosafe cast. Having taking
careful measurements after waiting the recommended 1hr you’ve
decided to keep the cast for future measurements. In that case, be
sure to document the date and time the cast was made. I apply Scotch
tape around the middle of the cast and write the info on it with a
permanent ink felt tip pin. I’ve inadvertently mixed up casts of the same
caliber and find this to be a better method than labeling a storage
envelope. Better yet, do both.
Adjusting Data from “Aged” Casts
The following information on expansion characteristics is from my
research and experiences in making well over 50 casts to perfect my
casting technique and gather chamber and bore information for my book
on the Browning BPCRs. I still have the original casts which were made
several years ago and have taken additional data months and years later
that directly correlates back to the original cast dimensions.
I mentioned Buffalo Arms and Brownells as two suppliers of Cerrosafe.
The cast instructions provided by Buffalo Arms does not discuss
expansion characteristics, but Brownells’ data sheet does discuss it.
Based on my experiences, I recommend that you completely ignore the
expansion comments provided by Brownells for two reasons. It became
clear to me that Brownells’ data did not correlate to the actual data I
measured over time. Plus, after discussing the discrepancies with a
Brownells technical representative, he could not substantiate the data
and only confirmed that the measurements should be taken 1hr after the
cast was made, essentially indicating that their expansion data may be
incorrect. Since then I have definitely confirm it's not correct.
Although I have not purchased Cerrosafe from MidwayUSA, early on,
as I was becoming familiar with the process and found that Brownells’
expansion correction factors were not correct, I shared my frustrations
and findings with a shooter very experienced with using Cerrosafe. He
referred me to MidwayUSA’s instructions and data sheet on the product,
which contained the accurate expansion factors listed below.
Contraction & expansion factors versus time, measured in inches per
square inch: (Note that the factors are referenced to the suggested 1 hr
measurement point)
• 2 minutes -.0004"
• 6 minutes -.0007"
• 30 minutes -.0009"
• 1 hour .0000"
• 2 hours +.0016"
• 5 hours +.0018"
• 7 hours +.0019"
• 10 hours +.0019"
• 24 hours +.0022"
• 96 hours +.0025"
• 200 hours +.0025"
• 500 hours +.0025”
So, other than to confirm that Cerrosafe does in fact initially contract
then expand as it cools and ages over time, how useful are the actual
factors. To start with, notice that the maximum shrinkage listed is at
30 minutes, which is the ideal time to remove the cast from the
chamber. Also note that the sign of the factor changes from negative
to positive 1 hr after the cast is made (the point in time when the cast
dimensions match the chamber dimensions). It may not be clear to
you now, but the sign is important when the factor is used in a simple
formula to adjust data taken from aged casts. Also note that at 96
hours (4 days), Cerrosafe has reached its maximum expansion point
and stabilized; years later it will measure the same as it did at the 96 hr
point.
Another key to using the data is noted in the table heading indicating
that the contraction & expansion factors versus time are measured in
inches per square inch. So now you’re probably scratching your head
and thinking, what in the devil does that mean? It certainly wasn’t clear
to me at first. What it actually means is, in order to use the correction
factor, it must be subtracted from the square of the measurement,
then take the square root of the result to obtain the corrected data.
For example, the cast neck diameter from my Browning .40-65 BPCR
chamber measured 0.4353” one hour after taking the cast. 24 hrs later
the cast neck measured 0.4375”. So, 0.4375” squared equals
0.19141”. Next, the appropriate factor (0.0022”) is subtracted from
0.19141”, which equals 0.18921”. Now, taking the square root of
0.18921” equals 0.4350”; a difference of only 0.0003” from the original
1 hr measurement. Now that’s real close.
Another example, the cast groove diameter from my Browning .45-70
BPCR measured 0.4590” one hour after taking the cast. Well over 96
hrs (actually 4 years) later the groove diameter from the cast measured
0.4615”. So, 0.4615” squared – 0.0025” = 0.2105”. Now, taking the
square root of 0.2105” = 0.4588”; only 0.0002” difference from the 1 hr
measurement, which is better than my measuring ability.
By the way, you might be thinking to yourself that the factors and math
may work fine to convert aged measurements to the 1 hr data, but how
can you be sure the 1 hr measurements accurately reflect the chamber
and bore dimensions? I had the same concern and verified several
chamber and bore dimensions using soft lead slugs. The Cerrosafe
measurements, taken at approximately 1 hr after the casts were made,
were “right on” within my measuring ability. I also used impact
impressions (discussed below) to measure chamber lengths and
compared the data to measurements taken from Cerrosafe casts.
The data agreed in all cases.
Advantages & Disadvantages
So what are the advantages and disadvantages to using Cerrosafe?
One advantage is the relative hardness of the cast. It’s not as easily
damaged when taking measurements as is an impact impression. If the
cast is sufficiently long to include a short section of the lands, it will
provide dimensional data of the throat and bore, but the same benefit
applies to the impact impression. Also, a Cerrosafe cast provides
diameter data along the entire chamber, including the center and rear
portion of the chamber; data which is not available from an impact
impression. For most shooters this data is not particularly important so
I don’t consider this much of an advantage, if any.
Disadvantages include: handling concerns & hazards, dealing with
expansion characteristics, having to partially or fully disassembling the
action to pour and remove the cast, and not being able to directly
measure case overall length (OAL). The latter two are the main
reasons I prefer an impact impression.
Chamber Mouth & Throat Impact (Swaged) Impression
In lieu of making a chamber cast with sulfur or Cerrosafe, I prefer and
highly recommend the following technique, which is also commonly
referred to as making an impact casting or impact slug.
Making an Impact Impression
Starting with a fire-formed case, which has not been resized, shorten the
length by approximately 0.200” (the amount is not critical) and fill it with
lead to approximately ½” below the mouth.
You’ll need a sufficient amount of pure or nearly pure lead slightly
smaller than the bore diameter. It will be dropped down the bore and
compressed to form the impression. If bullet lead is not available, soft
lead wire can be found in the fishing section of most sporting goods
stores.
Clean, lightly oil the chamber and insert the partially lead-filled fire-formed
case.
Next you’ll need a steel rod just under bore (land) diameter, available at
most hardware stores. The rod should be a few inches longer than the
barrel with a squared-off cleanly finished end. Wrap the squared-off end
with tape to make a snug fit to the bore, which will help in centering the
end of the rod and eliminate any possibility of the edge galling against
the bore. If a rod of the proper diameter is used, the possibility of
damaging the rifling lands is just about nonexistent. Taping the end that
contacts the impression is just an extra precaution in case the squared
off end is irregular or not properly finished. The only other location I'd
consider taping is the section that comes in contact with the muzzle to
protect the crown.
With the rifle standing up, butt stock on the floor; drop a sufficient
amount of lead down the muzzle to fill the chamber and throat when the
impression is completely formed. Be conservative and don’t use too
much lead or the impression will be too long, extending down the bore
and hard or impossible to remove without damaging it. If, after
removing the impression, you find that the amount of lead was not
sufficient, it’s easy to redo with the addition of more lead.
Using a piece of paper towel, insert a wad of around 1/2" into the
muzzle end. The paper will compress, fill the grooves and prevent the
rod from embedding itself and seizing on the end of the compressed
lead impression.
Insert the wrapped end of the rod into the muzzle until it contacts the
paper wad. Now strike the rod with measured even stokes, just enough
to fully compress the soft lead. When the lead is fully compressed you'll
feel the difference as the rod will start bouncing with every stroke of
the hammer.
Open the action and gently push the case and lead impression out with
the steel rod. Fold up a soft cotton rag or towel to catch the impression
as it’s pushed out of the chamber. Be gentle so as not to further deform
the lead impression. If the correct amount of lead was used, the
impression should neatly form to the dimensions of the neck and mouth
of the chamber, the throat and a small section of the bore. If it’s not
long enough, repeat the process after adding more lead.
Note - When adding additional lead to an existing impression, I've had
good success with roughing up the end of the lead impression,
reinserting it, adding additional lead down the bore and swaging a new
impression. In all but a couple of cases the new lead firmly adhered to
the old impression and the seam was virtually invisible. The couple of
times the added lead did not stick well were most likely due to oil
contamination. If, after roughing up the end of the old impression, you
still have a problem with the added lead adhering, essentially start over
by cutting off the lead extending out of the case and drill a little out of the
mouth to a depth of 0.100” to 0.200”, or heat up the lead-filled case and
pour a little out, which may be the easier way.
Advantages & Disadvantages
There are several advantages using this technique. It’s cheaper, simpler
and easier than using Cerrosafe. The rifle action does not require
disassembly and there’s no chance of spilling excess sulfur or Cerrosafe
into portions of the action. Measuring the impact impression from the
rear of the rim to the start of the transition step includes the actual
headspace clearance of the rifle and will provide the correct OAL of a
properly trimmed fire-formed case. Plus, if the impression is sufficiently
long to include a short section of the lands, it will provide dimensional
data of the throat and bore. Finally, there’s no concern about the
impression changing dimensions over time.
There are a couple of disadvantages which I feel are not significant, but
should be discussed. Being made from soft lead, one negative is the
ease of damaging the impression or slug if it’s not handled with
reasonable care. Of course, if damaged, it’s relatively simple to make
another one. Another disadvantage is the inability to accurately measure
diameters along the center portion and rear of the chamber; data that's
generally not required or useful.
Wishing you great shooting,
Wayne