The weird silencer on the shotgun in No Country for Old Men may or may not be realistic, but I have to say that was one of the most gun-oriented movies I've ever seen, not in the sense of being filled with shootouts but because the characters were constantly interacting with guns on an intimate level in close camera shots - checking the chambers and magazines of found pistols, adjusting the scope on the rifle, sawing off the barrel of the shotgun in the hotel room and even smoothing the cut with a file and wrapping duct tape around the grip, and the Sheriff even talks about all the different calibers of the fired casings that he finds at the crime scene.
The novel is the same way, it goes into great detail about all the specific guns.