I have a recently made mcr, running on a registered Colt lower. My lower is rigle length, not a carbine, so I use a machined aluminum spacer in the buffer tube. I use loosened links with a dab of oil. Break in took a while with m855, but now it runs very well with m193. I did screw up and failed to clean the gas piston. After thousands of rounds, the piston housing eventually blew up. What a dumb ass move on my part. Flightlite fixed it for free, it took 7 months. They would not return any calls or emails, I couldn't get ahold of anyone, ever, after countless calls and emails. I thought my upper was gone for good, then one day fed-ex dropped it off. Back to being a happy, (and slightly wiser) shooter.