I had two issues with my second AR build on it's first firing. I built both the upper and lower a few months ago, but just now got a chance to fire it. First I tried to use a full P-Mag, and the mag would not go all the way in and lock in placed when the bolt was closed. The top of the mag was hitting the bolt, keeping it from locking in place. I pulled the charging handle back far enough so that the bolt cleared the mag well, then tapped the mag in place. I then had nothing but failure to loads from that mag. At times it would strip a cartridge, but it jammed on it. I moved to a Colt brand mag, and had the same problem with it not locking in place in the mag well unless I pulled the bolt back and then tapped it in place. Once I did that, everything loaded and fired from the Colt mag. No FTF, or FTE with that one. That tells me the P-mag brand mag needs to go to the trash, but what do I need to do about the mags not locking in? When I load an unloaded mag, they lock in just fine, but not when they're loaded with cartridges.