Well I tried different magazines,The 20 rounders that load on the right hand side first,the bolt will not lock open on.The 30 rounders that you load on the left side first,they do lock open on??I was using H4198 when it would not feed correctly.I used H335 and CFE 223 and it feeds correctly as long as I use the 30 rounders.I have about 140 rounds through it now.I even put that upper on another lower and it was better,but still did the miss feeds.I will admit I do not know a lot about AR`s.I have two 223 wyldes that I have run thousands of rounds through with no hiccups.Go figure?
