Local walmart had a bunch of Remington .223's with 50 grain HP's that weren't moving (most of the guys using .223 around here use them for coyotes or deer) and had them on sale for less than new brass prices. I bought all they had with the intention of plinking to empty the brass, then reload with my regular loads. At some point I ejected an unfired round from the AR I was using and that 50 grain HP was very noticeably pushed back into the case neck. I manually cycled a few more and about every third or fourth one would do it. From then on I only shot them in my bolt gun.
I realize this is factory ammo but this just reinforces my belief that if you are using reloads in an AR, you should crimp the bullet in the case.
kwg
