Here's the Stevens Ranger 16ga I bought. It isn't 100% correct to Max's shotgun, but is still very close, and the price was right:

Ok, I didn't buy a real sawed off shotgun. That's just a photoshop job I did on what it WOULD look like. The barrel on mine is one inch longer than the legal limit, and mine has the full stock. Here's how it looks:

Has anyone carefully looked into what kind of shotgun Max had?
I've read about the Stoeger Coach gun, and it does look kind of close. Also, the Stevens Savage 311 and 411, they look close too, and like I said, my Stevens Ranger even looks pretty close.
But Max's shotgun in MM1 looks like it has a little white band and a black cap at the bottom of the handle, and mine doesn't have that. The Stoeger Coach and Stevens 311 and 411s that I've seen don't have that either.
It also looks like Max's gun has a diamond shape cut into the front grip. That can be seen around the time he shoots Bubba off his bike.
Anyone have any other thoughts as to what it might be?