I love movie guns threads... Just to share some observations from spending time with a movie armorer (a good friend of mine): nobody, except some small minority group of gun-nuts will ever question that SBS abomination. Almost everybody in a film crew (including big name action stars) never shot even a single live round thru a firearm. Actually, most of them are really anti-gun and they freak out when you explain to them that those ARs over there are actually "real" rifles, only with a blank adapter. Armorers don't have the right of opinion to what looks real or not, they only make sure that everything is safe if there is a shooting. For the reality factor they got "weapon supervisors"... And they are the reason you see a 5 sot snubby shot 30 times without reloading, or a hand held Browning .50 BMG blowing cars 50 feet in the air...
Fact - you have a bunch of highly paid guys (and girls), that have zero experience with real firearms, to make an action movie. Not every time, but most of the time.
Boris