I don't know much about ARs, as I do not own one. I do have a NM M1A and I have shot thousands of rounds through it that I have loaded. I have never used a crimp on my rounds and I have never experienced any problems as a result. I have even fired a couple of rounds and measured the rounds in the magazine to check if their O.A.L. had changed. My loads have always chambered just fine. I usually use the 168-grain HPBT bullets made by Sierra and Hornady. I have talked with many other shooters who load for their M1As and some of them use a crimp and some don't.