Looks like he used the parts of a Thompson, as per the original.
However I wonder if he really mangled a spas 12 to fit it into the assembly or did what the prop makers did and take the action of another shotgun and replicate the housing.
Either way, sounds like an expensive and time consuming job.
Nope, never mind. He converted it to 9mm and utilized some high cap finnish magazine to better simulate the rather impressive ammo capacity of the guns from the movie.
Hey man i think you're wrong i think he used the upper reciever from an m24 and the gas block from a Uteba, then probably managed to spot weld grandpappys duck gun underneath it, and judging from the sound that gun is making, I am going to have to call bullshit and say it is beyond any shadow of a doubt chambered in 8mm mauser.