With the barrel marked 5.56 only, it is not a Colt barrel that was cut down.  A true colt barrel would be marked CMP, and if cut down to 11.5" would have no markings since they would have been on the section cut off.

Since the barrel is 11.5", the Flash suppressor is permanently attached, and if you did remove it, you would still have to permanently attach another unit of the same length to meet the minimal barrel length of 16" in order for the rifle to no be a NFA registered weapon (Short Barrel Rifle).