This is definitely not true. I only use FL die sets for 5.56 and .308 Win, and don't have any issues running my ammo through any 5.56 or .308AR that I have. No issues running that same ammo through any 5.56 and .308 Win bolt guns that I have, either.
^^^ That's it, right there. I learned that mistake when I first started making my own 300BLK brass. Not much shoulder on those little bastards, and if you add too much crimp, you swell it, and it doesn't chamber. Well, not the first time - You have to repeatedly beat it into submission, and it'll chamber. :bat: