So I'd love to just buy a Colt LE 6920 M4A1 SOCOM, unfortunately I live in the people's republic of NY, so I'd have to build it with a fixed stock and a "muzzle break". I was thinking about using a DD CHF barrel, maybe even a 14.5" with a permenantly attatched muzzle device, not sure if it's worth the trouble. My two main questions are what kind of upper does Colt use? I know my M4 Match Target has a RR upper, but I'm not sure about the LE 6920. And which one of Knight's Armament rails does Colt use and is there anything I should keep in mind with those. For example; are they compatible with DD barrels and whatnot.