TexasMac's Web Site
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
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 in 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,