Here we go again! The search function will turn up about a dozen threads on this subject, but I'll once again give a short tutorial on the use of small pistol primers in the .45 acp.
The use of small pistol primers in .45 started (at least in this country) when the quest for primers without lead styphnate started several years ago. This was prompted by the increased use of indoor ranges and people concerned with lead poisoning. The quest is ongoing, by the way, and I was told by a technician at the ATK booth at the SHOT Show in Las Vegas in January that it wouldn't be long before all primers would be Non-Toxic, and not contain lead.
Anyway, the Non-Toxic (NT) priming compound, which is an explosive, just like lead styphnate, has a higher brisance than the lead styphnate compound. Brisance is the velocity of the explosive gases. The increased brisance made the primers back out of the primer pocket faster than lead styphnate primers, so they would expand outside the primer pocket before the burning powder inside the case had a chance to produce enough pressure to push the case back and reseat the primer. (this is what occurs when a cartridge is fired) This caused excessive flattening of primers and the brass primer cup would flow.
The first attempt to fix this problem was enlarging the flash hole to almost 1/8" in diameter. This was done by both Federal and Winchester. These cases will load just fine, by the way.
The second attempt at a fix was to add a heavy crimp to the primer pocket, after the primers were seated. This is similar to what's done on military ammunition intended for use in full automatic firearms, which keeps a primer from exiting the primer pocket and jamming up the works, usually at the worst possible time.
Neither of these fixes proved fully satisfactory, so the next iteration was the use of the small pistol primer. The volume of the .45 acp case doesn't really require a large primer anyway, but "we've always done it that way". The small pistol primers work just fine, and some reloaders have reported increased accuracy when using the small pistol primed cases.
The Bushmaster reported he got about 50 fps less velocity when he conducted some tests on the two different primer size cases, but no loss in accuracy. I've also found no loss in accuracy, but haven't bothered to run them across the chronograph to measure any possible velocity differences, since they all hit at the same point on target.
I've seen small pistol primed .45 acp brass from Winchester, Federal and Speer. It's not going away, so you might just as well sort it out and reload it. Repeat as necessary.
Hope this helps.
Fred