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Last update: 3/15/13

Table of Contents
Introduction
Quick Methods to Determine Case Length
Quick Method 1
Quick Method 2
A Few Words on Sulfur Chamber Casts
CerroSafe Chamber Casts
Comments & Cautions
Making a Fusible Alloy Chamber Cast
Adjusting Data Taken From “Aged” Casts
        Currently Published Data is Incorrect
        Experimental Details Overview
        Correctly Adjusting Data taken from “Aged” Casts
Advantages & Disadvantages of CerroSafe Chamber Casts
Chamber Mouth & Throat Impact (Swaged) Impression
Making an Impact Impression
Advantages & Disadvantages of Impact Impressions


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 gap width and the position of
the bullet driving bands and lube rings in relation to the gap, the gap may fill with
lube or lead from the expanded bullet.  Assuming the gap is filled with lead, as the
bullet moves forward the lead will either be smeared back onto the bullet or
stripped off and deposited in the transition step as a lead ring.  If the gap is
excessively wide due to a very short case, such as firing .45-70 ammo in a .45-90
chamber, the displaced lead may be smeared back to the base edge, forming an
irregular cupped or “finned” base, which is certain to degrade accuracy.  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 case overall length (COL)
of a fire-formed (fired but not resized) case 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 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 stretch, if any,
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 are two quick methods that are not as accurate
as other techniques, only provide the approximate case length but should be
sufficient for most shooters.  The third and forth method consists of making a
chamber cast using either sulfur or a fusible alloy.  The final method discusses
making an impact impression of the chamber and throat and is the technique I
prefer to use and highly recommend over the others.  All will work, but there are
benefits and tradeoffs associated with each technique.

Quick Methods to Determine Case Length
Neither of the quick methods discussed below will provide diameter dimensions
of the chamber, throat, or bore.  And both techniques require a case that is longer
than the chamber being measured.  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
for this exercise.  One example is reforming .45-70 Gov. cases to fit into a .40-65
Win. chamber.  Another option is to use a 45-90 case for a .45-70 chamber.

Quick Method 1:
As already noted, this method requires a case longer than the 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, then allowed to cool.  To quickly cool the neck
and speed up the process a bit, knock it over into the water.  Now lightly 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 chamber to bore 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 up as shallow impression on the end of the case, but I do not
recommend measuring them to determine bore diameter dimensions.  “Slugging”
the bore with soft lead will provide accurate measurements of the groove and bore
diameters.

Quick Method 2:
The following technique also requires a case longer than the chamber.  Expand and
then flare the mouth of the case so you can feel the flared portion dragging on the
inside of the chamber wall when it’s being pushed in.  Continue to push until it
comes to a hard stop, which means that the mouth or lip of the case has hit the
start of the chamber-to-throat transition step.  Measure the amount of the case that’
s sticking out of the chamber.  Subtract the amount from the overall length of the
case.  The result is the maximum case length for your chamber.  
Note: Typical chamber-to-throat transition steps can have a step-angle of 45
degrees or less.  If the angle is very shallow, i.e. a few degrees, this method may
not work very well.

A Few Words on Sulfur Chamber Casts
Earlier I mentioned using sulfur to make a chamber cast.  Prior to the development
and use 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.  
Although high purity pharmaceutical grade sulfur is not required for a chamber
cast, it is available in several forms.  Flowers of sulfur, also known as sublimed
sulfur is made by sublimation and is the most common.  Two other forms are
precipitated sulfur, also known as milk of sulfur, and washed sulfur.

There are several negatives associated with using sulfur for a chamber cast.   It’s
somewhat dangerous to melt and can ignite with an almost invisible flame.  It can
be very messy and hard to clean out if the chamber is overfilled and the sulfur
runs into or is accidentally spilled into the action.  And sulfur casts are very
brittle.  I have not used sulfur, but understand the process is very similar to
making a CerroSafe cast, which is discussed later.  Roy F. Dunlap 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.  Following is one
shooter’s technique I found during an Internet search.

The chamber and a portion of the bore should be clean and swabbed with a very
light coat of lubricant or WD40.  From the muzzle end, push a tight patch down the
bore to about ½” ahead of the throat.  After removing the stock and/or forearm,
mount the well padded receiver and barrel in a vise.  Slowly melt the sulfur over
low heat, being cautious that it does not catch on fire.  Heat the barrel and the
chamber area with a propane torch or hot air gun until it’s just too hot to hold.  
Plug the extractor slot.  Now carefully pour the melted sulfur into the chamber all
the way to the top. A funnel shaped depression will appear in the center, which is
normal. Wait several minutes for the chamber to cool some and gently knock the
cast out with a rod from the muzzle end.  Be sure to catch it.  If measurements are
taken immediately, the cast will be about .0001” smaller than actual chamber and
bore dimensions.  Within a few hours the cast will not shrink more than .0005”,
which is certainly close enough.  After more time has passed the cast will
eventually shrink .001”.  Here’s a YouTube video on making a chamber cast using
sulfur:
http://www.youtube.com/watch?v=j8jE2o7ePbo.


CerroSafe Chamber Casts

Comments & Cautions
Making an impact impression or a chamber cast are recommended methods for
BPCR shooters to determine the actual chamber and throat dimensions of a firearm
to use as a reference when reloading accurate ammunition.  Chamber casts are
typically made using a “fusible alloy”, which is the most common method in use
today.  The term “fusible alloy” generally refers to a metal alloy that melts at
relatively low temperatures; typically below 361°F (the melting point of lead tin
eutectic solder).  The most well known fusible alloy for chamber casts is sold
under the CerroSafe brand.

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 currently produces it.  Until 2007, CerroSafe was made by Cerro
Alloys, hence the CerroSafe brand name.  Bolton Metal Products acquired Cerro
Alloys from Cerro Metal Products in February 2007.  The fusible alloy is now
called Bolton 160-190° in reference to the melting range, but it’s still supplied in
½ lb ingots under the CerroSafe brand.  Bolton Technology is headquartered in
Bellefonte, Pennsylvania.  Although RotoMetals, another manufacturer, calls their
version Chamber Alloy 158-190, throughout the following discussion I will refer to
the alloy as CerroSafe for simplicity.

CerroSafe 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 158°F to 190°F.  It’s available from gunsmith
supply companies and several firearms accessory retailers in ½ lb or larger
ingots.  I’m currently aware of five retailers.  Brownells, MidwayUSA, Buffalo
Arms and Track of the Wolf sell CerroSafe supplied by Bolton Metal Products.  
RotoMetals manufactures their version of a fusible alloy using the same alloy
formula used in CerroSafe.  Per equivalent weight, RotoMetals current price is
about 1/2 that of CerroSafe.  There may be other suppliers.  The typical fusible
alloy 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 more than sufficient to make two casts.
































A few words of caution are warranted concerning handling an alloy containing lead
and cadmium.  We are all aware of the harmful result of inhaling or digesting
lead.  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.  To give you an example, the
permissible exposure limits (PEL) of lead is 0.05 parts per million (ppm).  For
cadmium it’s only 0.01ppm.  Repeated or prolonged exposure to cadmium can
damage the kidneys, lungs and/or liver.  So use extra caution and be sure to wash
your hands when melting and handling CerroSafe or similar fusible alloys, or casts
made from fusible alloys.

If all you need is one or two casts and have no plans to measure the cast
dimensions again, ignore the discussion on CerroSafe’s expansion rates except for
the important 1.5 hr measurement point.  But if you plan on keeping the cast for
future measurements, than a clear understanding of the expansion characteristics
are necessary to ensure the data accurately represents the true dimensions of the
chamber, throat and bore of your rifle.  An in-depth discussion on expansion
characteristics follows the details on making a cast.


Making a Fusible Alloy Chamber Cast
The following steps detail how I make a chamber cast.  No doubt there are many
variations used by others.  For one example, here’s a YouTube video made by
Midway USA’s Larry Potterfield:
http://www.youtube.com/watch?
v=2d5DA4_OcuE.  Larry leaves out some of the details but the video will give
you a general idea of the process.

It’s well known that once the cast is poured CerroSafe shrinks and then starts to
expand over time as it “ages”.  Some suppliers note that the cast will match the
chamber dimensions 1hr after the cast is made.  In a controlled experiment
discussed below, I have determined that 1.5 hrs is a better estimate of the ideal
time to take measurements.  Measurements taken before or significantly after 1.5
hrs 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 30 minutes to an
hour will if precise measurements are required.  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 preferred time interval you’ll have to make
adjustments to the data (see later instructions) or remake the cast.

Although disassembling the firearm 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.

•        Disassemble the rifle as needed to gain access to the chamber.  On many
single-shot rifles it is only necessary to remove the forearm in order to heat the
barrel chamber and throat.
•         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, throat and at least a short section of the
bore forward of the throat.
•        Fold up a cotton cleaning patch or section of paper towel and roll or form it
into a cylinder shape until it will snugly fit into the bore and act as a plug/dam for
the melted alloy.  I prefer to insert the patch from the muzzle, but it can also be
inserted from the breech or chamber end of the barrel.  Using a cleaning rod, force
the patch down the bore until it reaches a point approximately 3/4” to 1” in front
of the mouth or throat of the chamber.  Ideally you’d like to see an inch or so of
the rifling when the cast is made..
•        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 125
degrees is about right if that gives you a better idea.  Do not overheat or get it so
hot that it’s uncomfortable to hold by hand.
•        There are many methods to heat CerroSafe.  You can melt it 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 heat resistant surface such as a brick.  
•        “Vice-Grip”-type locking pliers can be used as a handle for the can.  Since
the melted CerroSafe should not be much more than 190 degrees F (less than the
temperature of boiling water), I prefer to use heavy leather gloves when handling
the tuna can.  It provides for better control while pouring.
•        Don’t forget to plug the extractor slot or you’ll end up with a real mess
when the alloy cascades down the slot into the action or receiver.  A common 6”
wood shaft cotton tip applicator works great for plugging the slot.
•        Slowly pout the alloy, completely filling the chamber until the alloy covers
the rim seat and forms a slight mound at the breech.  I’ve found that pouring too
fast may result in very large voids in the cast.  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 or hot air gun while
pouring.
•        Wait approximately 10 minutes for the cast to solidify and shrink to push it
out of the chamber.  The maximum shrinkage time is 20 to 30 minutes after which
it will start to enlarge.  Waiting any longer and the chances increase that the cast
will not come out as it further enlarges.
•        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.  If you can’t catch it by hand or miss, the towel
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, reposition the rifle with the breech up and 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 heat the
barrel sufficiently to melt the alloy and pour it out of the barrel.
•        With the cast in hand, check out the surface condition.  Some small voids
and frosting is normal, but many wrinkles or large surface voids are an indication
that the alloy was not hot enough, the barrel was not sufficiently warm or the alloy
was poured too fast into the chamber.  Heavy frosting on the surface is an
indication of overheating of the alloy or barrel.  If either of these conditions
prevents good measurements, make a new cast.

So, now you’ve successfully made a CerroSafe cast.  Having taking careful
measurements after waiting the recommended 1.5 hrs 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 Taken From “Aged” Casts
Most shooters make a chamber cast, take some measurements and never use the
casts again for various reasons.  The most obvious reason is, once accurate
measurements are made and documented at the correct time, there’s little or no
need to revisit the cast.  Another reason is they are aware that the cast will expand
over time, but are unsure how to determine the original dimensions from the aged
and expanded cast.  For several reasons I’ve had the need to use old CerroSafe
casts to verify original measurements and/or take dimensional data from different
locations than originally measured.  In doing so I came to the conclusion that the
published expansion data are flawed as is the method to use it.  
The bottom line
is the expansion data that has been published to date is very old (dating back
to 1934) and I experimentally determined it to be incorrect.

Currently Published Data is Incorrect
I mentioned Buffalo Arms, Brownells, Midway, Track of the Wolf and RotoMetals
as suppliers of CerroSafe or CerroSafe-like alloy.  The alloy instructions provided
by Buffalo Arms and Track of the Wolf does not discuss expansion
characteristics, but Brownells’ data sheet does discuss it.  Bolton Metal Products,
RotoMetals and MidwayUSA provide a list of the old incorrect expansion factors
on their web sites.  See Table 1 below.  Although RotoMetals manufacturers their
own alloy, they admitted to using the old data originally provided by Cerro Metal
Products as does MidwayUSA.  

Not only are the incorrect expansion factors listed by Bolton Metal Products,
RotoMetals and MidwayUSA, the method of applying the factors remains
incorrectly listed in inches per square inch by Midway USA.  After researching
their old records, Bolton Metal Products assured me the original published factors
are in inches per inch.  After I alerted RotoMetals to the discrepancy, they recently
updated their data to agree with Bolton Metal Products.






















Brownells does not list the expansion factors in their CerroSafe instruction sheet
but says the expansion amount will be .0025” after 200 hours.  Note that this is an
absolute value in inches and is not a mathematical factor.  After carefully
measuring several casts it became clear that Brownells’ data did not correlate to
the actual data I measured over time.  Subsequently a Brownells technical
representative could not substantiate their data, only confirming that the
measurements should be taken 1hr after the cast was made and conceding that
their 200 hour expansion amount may be incorrect.

Experimental Details Overview
The following information is from my experimenting, research and experiences in
making well over 50 CerroSafe 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.  Since then I’ve made
several more casts, but more important, after realizing the published expansion
factors were incorrect, I felt the need to verify the actual expansion characteristics
of CerroSafe.  To determine the correct aged cast expansion factors I recently
conducted an in-depth experimental analysis.  Applying the experimental results to
the old casts yielded the original chamber dimensions as verified by lead slugs and
impact impressions.  During and after the experiment was completed I discussed
and shared the data and experimental details with Bolton Metal Products.  They
did not dispute the results but, to date, have not changed the factors listed on their
web site.  Hence, the same incorrect factors also remain on the web sites of
RotoMetals and MidwayUSA.

CerroSafe casts were made of two chambers, .45-90 and .40-82.  Prior to taking
data, both ends of each cast were filed smooth and locations were identified and
marked to take length measurements.  The casts were slightly longer than 3”.  As a
reference to determine how much the CerroSafe casts actually expanded in
diameter over time, lead slugs were used in the chamber neck, the freebore in front
of the chamber to throat transition step and at a short distance forward of the leade
in the bore.  The slugs were driven into the chamber, throat and bore, and
expanded with metal rods from both ends to ensure they fully expanded.  They
were then gently removed.  The 1st measurements were taken 1 hour after the
casts were poured.  As they “aged” the length and diameters were closely
measured at the same locations the lead slugs were made.  Measurements were
taken to an accuracy of one-ten-thousands of an inch (0.0001”).

What I found was, by the time the casts had aged 5 hours all the resulting
expansion factors were much larger than previous published.  And it quickly
became clear that the diameter expansion rates were larger than the length
expansion rates, which was a surprise.  More important, the cast dimensions
matched the chamber dimensions at approximately 1.5 hours, not at 1 hour as the
old data indicated.  Finally, as indicated by the results, it’s important to note that
by the 200 hour point the casts had stabilized and did not expand further, at least
not to a significant amount.

Correctly Adjusting Data taken from “Aged” Casts
Based on the experimental data, the expansion factors listed in Table 2 are what I
now use.  The factors are in inches per inch.  Therefore, to calculate original
dimension (X) from aged dimension (Y) use the formula X = Y / (1 + Factor).  For
a close approximation use X = Y (1 - Factor).  For example, using 0.461” for the
groove diameter of a 200 hour or older age 45 caliber cast will result in an
approximate value of 0.4587”.  The formula is X = 0.461” / (1 + 0.0051).




















Note – I have not experimented with RotoMetals’ 158/190 chamber casting alloy
to determine if the experimental CerroSafe data results apply, but would be very
surprised if it did not since the alloy is the same.

So, if you have been holding on to old CerroSafe chamber casts, pull them out,
take some careful measurements and apply the formula and expansion factors
listed above.  The results should approximately match the chamber, throat and
bore dimension of your firearms.

Advantages & Disadvantages of CerroSafe Chamber Casts
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 as an
impact impression, which is discussed below.  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 both the sulfur cast and 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, not being able to directly measure case overall length (COL) and,
depending on the design, having to partially or fully disassembling the action to
pour and remove the cast.  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 case neck, 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 .100” to .200”, or heat up the lead-filled case and pour a little
out, which may be the easier way.

Advantages & Disadvantages of Impact Impressions
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 head clearance of the rifle and will provide the
correct COL 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 for most shooters.


Wishing you great shooting,
Wayne
CASE LENGTHS, CHAMBER CASTS & IMPACT
IMPRESSIONS
By Wayne McLerran