Functionality difficulty:  To the third round I get a lightweight hammer strike.  I've experimented with the two H2 and H3 buffer weights.  H2 I was only getting 1 shot, H3 I now get two photographs off.  I have attempted this on two uppers, 1 carbine and one pistol, but utilizing the very same BCG.  I ordered an aimsurplus NiB BCG, I'm gonna