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