I had some really annoying failures when I first built my rifle (there's a sticky on my build and its problems in the AR-15 forum). After testing all sorts of things, it turned out to be the crappy Remington UMC ammo.
I'm not saying this is an ammo problem, but most problems are mags and ammo.
If this is a chamber problem--and that's what it sounds like--you really ought to have a gunsmith check out the headspacing. But you said the brass looked normal when it came out? Not bulging?
.45acp, .40 S&W, 9mm, .38 Spl, .380 acp, 22lr
12 gauge, .223/5.56x45mm, 30-30 Winchester
2 Chron. 7:14 Christians must seek His face