Does it lock back if you just run the charging handle with an empty mag in it? My M5 would not lock back on the last round, but it would lock back manually. It also ejected cases about 2 feet. I measured the gas port and it was way undersized, I want to say it was something like 0.080", but I don't remember for sure. I read that DPMS spec for an 18" RLGS was 0.093", so that's what I opened it up to. It locks back every time now. ETA, I'm using Magpul mags.