I have always head/read that 41s were the best way to prevent slamfires in semi auto riles. That said I have loaded and shot north of 10 k WSR primers in ar's and mini 14s and have yet to have a slam fire. Maybe I am just lucky, or maybe there is another issue. Try shooting some of your loads in another ar. If problem occurs it has something to do with the ammo. If not it is the rifle. Then try another bcg. My guess would be firing pin is binding in the bolt or possible has too much protrusion.