I might have built 15+ ARs and this is the first for me. When putting the lower together I put the charging handle and bolt in the upper to see how the upper and lower fit together. When trying to charge the handle, it would not go back over a 1/6". I ask myself what the hell is going on? Taking a closer look, the backing plate was sticking up about 1/32" or so too high. The charging handle was hitting the backing plate. Lucky, I have a couple extra backing plates on hand. Took the oversized one out, stuck it in a vise and filed it down a little. Will try it out on the next built.