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LARGE PISTOL PRIMERS & PRIMER WADS
IN BPCRs
By Wayne McLerran
Last update: 5/20/15

Going back a few years, 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, due to
experimenting by some well-know competitively successful shooters, it was
found that lowering the power (brisance) of the primer can directly contributed to
a significant reduction in the velocity extreme spread (ES) and standard deviation
(SD).  In many cases (no pun intended) the SDs dropped from double digits to
low single digit values.  I’m not aware of conclusive evidence as to why a weaker
primer reduces SD.  It’s been debated that a stronger primer can push the powder
column and bullet forward prior to full powder ignition, resulting in shot-to-shot
inconsistencies.

Large Pistol Primers
One method in common practice today to reduce primer brisance is to use large
pistol primers in place of large rifle primers.  For a chart comparing the power of
various primers go to
http://www.castingstuff.com/primer_testing_reference.htm.
Although the diameter is the same, large pistol primers are 0.008” shorter and the
cup material is slightly softer than large rifle primers.  In loads for most BPC
rifles this works fine, but be on the lookout for the following potential problems:
1.  Since rifle firing pins generally strike the primer with more velocity/energy
than handgun firing pins, pierced primers can result.
2.  If you’re not paying close attention when seating primers, the potential
increases of not fully seating a large pistol primer and not realizing it.  Although
it’s not a dangerous situation, misfires can result since some of the firing pin
energy is used to shove the primer forward rather than adequately indenting the
primer.
3.  If the primer is seated without a spacer to compensate for the difference and
primer height and the firing pin tip protrusion is marginal, a misfire or “hang fire”
can result.
4.  If the primer is fully seated without a spacer and fired, cartridge/chamber
pressures will drive it back into the breechblock face with more energy than a
flush-seated large rifle primer.  Although generally not a problem with modern
hard breechblocks, the force may be sufficient to eventually “peen” the area
around the firing pin opening of softer breechblocks found in some 19th century
or early 20th century receivers.

Primer Wads
Another method to reduce primer brisance is to place a thin paper wad over the
primer.  The wad can be made from newsprint, printer paper or similar material.  
The wad is positioned between the primer and the bottom of the primer pocket,
and is referred to as an over-primer wad (OPW).  Or a larger diameter wad,
referred to as an under-powder wad (UPW), can be inserted in the case prior to
loading with powder.

Over Primer Wads (OPWs):
OPWs are typically used with and cut out by the large pistol primer during the
primer seating step.  Thin paper is inserted between the primer and the case as the
primer is being seated.  Seating the primer cuts and seats the wad, covering the
flash hole.  Since the paper takes up some room in the bottom of the primer hole,
when fully seated the pistol primer is positioned further back, limiting the paper
thickness to around 0.008”, which is the difference in height between a large rifle
primer and a large pistol primer.  When using large pistol primers some shooters
use an OPW solely to eliminate the potential breechblock peening problems noted
above.  The wad thickness compensates for the shorter primer and also further
reduces the brisance of the large pistol primer.

Under Powder Wads (UPWs):
UPWs are normally punched out of paper and dropped into the case to cover the
flash hole prior to loading powder.  UPWs can be made with cheap hand-held
punches or a press mounted punch used to make the much thicker over-powder
wads.  When inserting the UPW, if a problem is experienced with the wad
flattening out against the inside wall of the case, a dowel rod or similar tool can
be used to push the wad down or use slightly smaller diameter wads.  Although
the standard cheap hand-held paper punch found at most office supply retailers
cuts a ¼” hole, they are available in other sizes.  I’ve used both a 5/16” and 3/8”
version for .40 and .45 caliber.  These will usually drop in and “flutter down” to
cover the much smaller primer hole.  The wad does not have to be centered as
long as the flash hole is fully covered.

One potential issue with UPWs is they may not be effective and taming the primer
power.  Based on recent testing I found it’s unlikely the primer will punch through
the UPW, but will push the wad and powder column forward sufficient for the hot
gases to be diverted around the edge of the wad and ignite the powder.  Also,
using 0.008” or thicker UPWs increases the chances of the wad remaining in the
bore and obstructing the following shot.  For more details and results from my
testing see the article titled,
BPCR Primer Wads Testing Results.

I understand that Steve Garbe discussed primer wads 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.  Some use a combination of large pistol primers and primer wads to
further reduce primer power.  Primer wads also serve to keep powder granules
from entering the primer hole and possibly causing erratic ignition.  Some
shooters believe this is more important than reducing the primer brisance.  
Although it’s been shown that primers will generally not blow through thick
wads, the wad thickness could have an effect on standard deviations.  Hence,
additional experimenting may be required to determine the wad thickness or
material that works best for your loads, or even if primer wads are beneficial.


Wishing you great shooting,
Wayne