LARGE PISTOL PRIMERS &
OVER-PRIMER WADS IN BPC RIFLES
By Wayne McLerran
TexasMac's Web Site
Here’s my “take” on Black Powder Cartridge Rifle (BPCR) reloading and using
large pistol primers and/or over primer wads.  At an earlier period in time, many
moons ago, the general consensus was that a “strong” primer was required to
reliably fire black powder.  Hence, it was standard practice to use magnum rifle
primers in BPCRs and the Federal GM215M Gold Medal Match Large Magnum
rifle primer was very popular.  More recently (around a decade ago or
thereabouts), due to experimenting by some well-know competitively successful
shooters, it was found that lowering the power (brisance) of the primer directly
contributed to a very significant reduction in the standard deviation (SD) of the
velocity spread.  SDs generally dropped from well into the double digits to
single digits.  And in some cases (no pun intended) the average velocity actually
increased slightly.  I'm not aware of conclusive evidence as to why a weaker
primer reduces SD.  It’s been debated that a strong primer can push the powder
column and bullet forward prior to full powder ignition, not a good thing.

For a chart comparing the power of various primers go to
http://www.castingstuff.com/primer_testing_reference.htm.  There are a couple
of ways to reduce primer brisance.  The most obvious technique is to use pistol
primers in place of rifle primers, but this can lead to problems with some older
rifles.  Another way to reduce or mitigate primer power is to cover the primer
face with a thin layer of paper or a similar material, commonly referred to as an
over primer wad (OPW).  Generally, once the case is primed, the wad is inserted
and pushed down with a dowel rod or similar tool to cover the primer hole.  The
OPW also serves to keep powder granules from entering the primer hole and
possibly causing erratic ignition.

I understand that Steve Garbe discussed OPWs in an article in one of his 2002
editions of The Black Powder Cartridge News.  Shooters have since
experimented with various paper thicknesses and used other more durable
material for primer wads.  Some use a combination of large pistol (LP) primers
with OPWs to further reduce primer power.  Although it’s been shown that
primers can blow through a thin newspaper print wad, the slight power reduction
may be sufficient.  Thicker wads that direct the primer flash around the wad
edges will also work.  Hence, if you decided to use OPWs, additional
experimenting may be required to determine the wad thickness or material that
works best for you.

I noted earlier that using pistol primers can lead to problems with some older
rifles.  The reason is due to the softer metals used in the breechblocks of older
19th century or early 20th century receivers.  Compared to rifle primers, pistol
primers seat slightly deeper or forward in unmodified rifle cases.  When fired,
cartridge/chamber pressures will force the pistol primer to slam back into the
breechblock face, which can result in “peening” the area around the firing pin
opening of softer breechblocks.  This is typically not a problem with modern
receivers and breechblocks.  If you do detect signs of peening you can
experiment with large rifle primers and thicker over primer wads or seat a large
pistol (LP) primer “through” a thin wad.  In other words, cover the LP primer
with a thin sheet of paper prior to seating it into the case.  The primer punches
through the paper creating an over primer wad and seats the primer further back
at the same time.

By the way, if you’re using OPWs that are inserted onto the case, use a smaller
caliber punch to cut out the wads or you’ll have a heck of a time pushing them
down to cover the primer as they will typically flatten out against the inside wall
of the case.  In other words, use a .38 cal. punch to make OPWs for .40 cal.
loads and a .40 cal punch for .45 cal. loads, etc.  These will drop in and “flutter
down” to cover the much smaller primer hole.

Wishing you great shooting,
Wayne