I just finished building an AR15 in 350 legend. I must admit that I did buy the completed upper. Building my own seemed to be cost prohibitive but I can already tell some of the things I want to replace. Anyways, I put around 40 rounds through it and had multiple issues with the bolt not completely closing after chambering the next round. After ever 1 or 2 shots I drop the mag and and the bolt closes and fires fine. I completely stripped and cleaned the BCG and and lubricated all moving parts. Is there something I'm missing? Could it be the mag? Or is it likely the ammo?