This seems a bit strange, since it's the length of a long gun that makes it more accurate and deadly. I thought maybe the fear is that a shorter gun is more concealable, but that doesn't make a lot of sense in a world with the Desert Eagle. Shorter shotguns are popular in the military for being maneuverable in close combat, so I suppose that could work to the advantage of criminals too. Probably best not to evaluate gun law on the basis of what makes sense. Michael Mann seems proud of his sound in "Heat", and the echoes booming in the L.A. canyons along with the tinkling of shell casings was eerie somehow. At least the guns don't click and rattle like someone walking through a pile of coat hangers every time someone handles one. (I assume that's the right link, I must resist the gravitational singularity of TVTropes today.)A sawed-off shotgun is specifically called out as a Title II weapon of the National Firearms Act.
most of what you see in film is synthetic foley