that shotgun

That was a SPAS 12 shotgun. It was made in Italy and is actually a pretty innovative, interesting platform that has been showcased in a lot of movies from the 80s and 90s and in a lot of video games (in general) for it's unique profile. It was manufactured between 1979-2000, though it is considered rare in American due to the assault weapons ban that was employed during the 90s.

The hook was designed so the operator could deploy the folding stock and then turn the hook to the side so he could brace it around his forearm, gaining leverage to fire the shotgun one-handed/one-armed. I would imagine this was conceived to be used in conjunction with a "shield" or to shoot around a wall/barrier while the wielder could remain mostly behind cover.

As this wasn't very practical I believe most people removed the hook, and tbh I think I've only seen it on the shotgun in this movie (probably to give the weapon a more menacing appearance).

The other feature that makes this shotgun unique is not only the duel functions of it being able to fire in both semi-auto or as a pump-action, but also to be able to activate either function on the fly by depressing a button that is located on the bottom of the pump grip.

The benefit of having either/or firing mode would be mostly to law enforcement. This would yield the ability to fire lethal, heavy loads such as slugs or buckshot as fast as one could pull the trigger, or use less than lethal rounds such as beanbags or teargas, which would not have enough of a powder charge to properly cycle the spent shell in semi-auto, and thus require the operator to pump it from the action.

However in movies I've noticed that they mostly opt for the actors to operate it in pump-action. Probably because it is more dramatic and attention grabbing to the audience.

[deleted]