Chargrow97

I've recently purchase and Aero Precision .308 AR-10 from a local gun store that assembled it for me. First, I had a problem with gun going into full battery and wouldn't fire, so I took it back to the gun smith and he removed an o-ring. Then, I tried it again after that and it would occasionally go into full battery, but then I would double feed every few rounds. I'm using Armscor M80 7.62x51 brass in it. What could the other problem be? TIA