I just put together a 9mm AR colt style carbine. I used the NFA ramped 9mm bolt, 7.5 oz buffer and a KAK magblock. Not sure who made the barrel but I got it from JSE.
After putting it all together I tested it with my reloads and it would fire 1 or 2 then click. No mark on the primer. I then switched to factory 115 federal and fired 30 rounds and had 1 FTF again with no marks on the primer. Anyone had this happen to them? Everything I've found online points to a different thing each time. I'm going to try a heavier buffer for 223 with some quarters since that was one of the suggests in older threads.