Meant to say AR-15, not AR-5 - fixed that in the previous post. Anyway, there can be feed problems in AR-15s, when you start mixing upper receivers and barrels that both have M4 feedramps, and don't. You said your rounds are catching in that specific area. First I've heard of it on a .308 AR, but that doesn't mean it hasn't happened before - I just haven't seen it yet, until now. Here's a pic of the problems with the AR-15 combinations (below) - I'd look at your feedramps, and the area where the barrel extension meets the upper receiver. If your rounds were catching there and causing Failures to Feed, then it's an issue.