I have a Colt 11.5" factory Colt 6933 upper (11.5" barrel) that I fitted with a .063" micro port from BRT to shoot with an Omega suppressor. One lower has an A5 RE, A5H2 buffer and Sprinco green spring. The other lower uses a carbine RE, H2 buffer and standard Colt carbine spring. The upper runs fine with either lower. If anything, it feels a touch over gassed and I'm thinking of trying the next size smaller micro port.