Last year I assembled a rifle based off of a Colt 6920. I ended up using a 13" CMR M-Lok and a factory Colt 14.5" SOCOM barrel. Due to the extremely thick profile of the barrel walls, the factory machined QD swivels at rear end of the rail were un-useable. I ended up having to buy an M-Lok QD mount (Magpul brand I think) to properly mount a sling.
I really like the Centurion rail and the SOCOM barrel is titties. I hope to build a backup clone rifle this year and the trooper looks like a great option Vs. an OEM version.