It's my 2nd post and I already need some help guys. I took a new SBR upper out today and started having problems from the get-go.

It's parts are: factory Sabre Defense lower w/ H3 buffer, Sabre Defense A3 upper, BCM Gunfighter, BCM BCG, Centurion Arms 12.5" barrel, Larue 11.0 rail, PWS QuickComp for a Gemtech G5.

The problem: Almost every time I fired, the BCG wouldn't strip the next round off the mag. It would extract the round fired, but wouldn't grab the next. I'd have to rack it each time to get a new round in the chamber. I also had several double feeds. I used a variety of USGI mags, PMags, and EMags and shot 55gr Wolf, 62gr Wolf, 55gr PMC, and 55gr Winchester putting a total of about 200 rounds downrange. I switched buffers to an H2 and even tried my friend's SBRed LMT lower and the problems still occurred. The only time none of this happened was when I used my G5 with the upper, during which it had no problems at all.

Any ideas what could be happening? I'm good at building rifles, but definitely need more experience diagnosing problems.