I see the weapon...
... but where is the mystery? an underfunded group of fellahin (that's Iraqi for "rednecks") could reasonably be expected to start making wepons themselves. Unless this is Action Man's gun, it something someone made theirself. Probably said "hey dudes, watch this" in Persian or Arabic before he fired it too. Me and my brothers and cousins made something like that once, we put black powder and rocks in it (ours wasn't shoulder fired, though). We were inspired to this act by an older pair of cousins who had done the same thing a generation or so before (one of them went on to become an engineer, the other one to operate an antiaircraft gun).
If I were going to take down a helicopter with one of those (and I'm not saying I will), I might consider putting some heavy gauge chain down the muzzle as a projectile. If it's a muzzleloader and not a recoilless job (you can make yr own rocket fuel with fertilizer and charcoal). Yeah, a barret .50 caliber might be better for the job, but they cost more than a fellah might make in several years.