Why Magnum primers?

[stokesrj]

I've personally seen four or five slam fires and in each case it was a CCI400 or Win SR primer. Remington even warns not to use their small rifle primer in AR's right on the box. Almost every year at the national matches at the command, "with one round load" at the start of the standing slow fire event, a round goes down range from a slam fire. It is usually a combination of things, thin walled primer, round inserted all the way into the chamber, and possibly a high primer. They all add up and even then it is a .01% issue. I've also seen blanking and pierced primers when folks run out of small rifle magnum primers and move to CCI400's especially. That is simply because they are on the edge and the ticker cup of the small rifle magnum primer was strong enough to withstand it and the small rifle primer is not.
I've never seen this issue in a 6.5 Grendel, these were all .223s.
Bob