This has been a discussion on many of the reloading forums for the past several years. Here is an explanation I posted on THR:
The demand for non-toxic priming has been long in coming, and has resulted in a lot of pressure on the ammunition companies to produce primers that don't contain lead. The long used standard priming compound has been lead styphnate, which has worked very well for a long time. It has a long shelf life, is stable and reliable, but releases vaporized lead into the air when detonated. Primers do detonate, since the priming compound is an actual explosive.
Numerous compounds were tried, and the one that was settled on is Diazodinitrophenol, aka DDNP for short. It's nearly as stable as lead styphnate, but has a much higher brisance. That's speed, or velocity, for those not familiar with explosives. The higher brisance results in more pressure, faster. This pushes the primer back out of the primer pocket much faster than lead styphnate priming does. When a primer detonates, it backs out of the primer pocket and the pressure from the burning gases of the gunpowder drive the case back against the bolt face, which reseats the primer into the pocket.
The DDNP primed cases showed extreme flattening of primers from this effect. This was caused by the fact that the primer was backed out longer than with the previous priming compound and flattened the primer cup by the time the case caught up with it. This can cause problems with some types of firearms, rendering them inoperable.
The first attempt to slow the DDNP primers down was crimping the primers in place. This was a marginal fix and didn't always work. The next attempt was enlarging the flash hole to relieve some of the pressure, which worked better. They found that combining crimping and enlarged flash holes pretty much cured the previous problems.
Along the way, they also found, or rediscovered, that the .45 acp case doesn't require a large primer to set off the small volume of powder in the case. There have been .45 acp cases primed with small pistol primers for many years, and quite successfully. The earliest one I have in my collection is headstamped 1957, but I've heard of much earlier examples, and I read one account that when John Moses Browning invented the .45 acp, he originally used a small pistol primer, but the Army insisted on large primers, so that was how he made them. I haven't verified this story, but it sounds plausible.
That brings us to the current era, where several companies are now making .45 acp brass with small primer pockets. I've seen Speer, Federal, Blazer, Winchester and at least one other brand, but I don't remember which one it was, so I won't hazard a guess and possibly make a mistake.
I've been loading both large and small primed .45 acp brass exactly the same for several years now. With the same data, except for the primer size, they hit the paper in the same place. Forum member The Bushmaster found that the same powder charge produced an average of about 35 fps less with the small pistol primers vs. large pistol primers with the same bullet.
Small pistol primers are nothing new, except in the U.S. Over the years I've seen many examples of small pistol primed .45 acp cases, but they were all of European production, and mostly Berdan primed.
There is no conspiracy by the ammunition companies against reloaders by changing primer sizes. The large primer has never been necessary in the .45 acp, and quite a few people who shoot mostly bullseye are reporting better accuracy at the 50 yard line with the small pistol primed brass. There is no cost savings to the manufacturer, since the small primer pocket means there's more brass in the base of the case, which adds up over many millions of rounds. The costs to make primers is basically the same for large and small, hence the same price for both from most companies.
I'm neither condoning, nor condemning the change, merely explaining it from the information I've gotten from the factory reps when discussing this issue with them at the various SHOT Shows.
Hope this helps.
Fred
You better believe I look now.
But Then I've got an old stockpile of 1000 pcs I've been reloading for years.
