If the upper is a Colt, it would appear to be from an
RO933 or "Colt Commando", which has an 11.5" barrel. Available on the commercial market in the semi-auto only model 6933.
Mostly stock 6933 with Ops suppressor mount
It could, however, be a 10.5" barrel as it would be pretty hard to make out a 1" difference in that picture. Frankly, it really wouldn't matter anyway.
There does also appear to be a
Gas Buster, which as posted above is generally used to divert blow-back gases out of the shooter's face when firing suppressed. The Gas Buster is also beefier all around and some people just prefer it.
While he doesn't have a suppressor mount, not all suppressors require a mount. Both the
Gem Tech Halo and the
Advanced Armament Omni can be mounted directly to the birdcage flash hider.
Not sure why the old stock, unless the upper was just added to an older gun or whatever, or maybe the shooter just prefers the old-style stock and added it to his new gun.