AIM 9mm NICKEL BORON BCG
I bought this BCG to replace the black one in my Foxtrot Mike upper receiver. It works great! However, there is one small problem with it. Because, what is called the "gas key" on AR-15's, is shorter than the one on the Foxtrot Mike BCG, I can't manually lock the bolt back. I think it's because now, the charging rod in the side charging hand guard doesn't come in contact with the BCG, and can't push it back far enough to let me engage the lever that locks the bolt back. If I had noticed that subtle difference I wouldn't have bought it. It's not any fault of the BCG or AIM.