It's some kind of Smith & Wesson revolver, very likely .357 Magnum, with a Colt-Python-style, non-standard-length barrel. I bet that the plating is not factory-standard, either, but I could be wrong about that.
Oh, yeah...and he's holding it wrong, too.
His shot's gonna miss, and its recoil is gonna put a hammer-spur dimple into his hairline.
What movie is it from?