Seems to be the standard pistol throughout the game. This weapon is seen being used by Vito, Joe and Henry throughout the game. The gun in the game is actually an M1911, close inspection of the backstrap shows that it is flat and not rounded.
In Chapter 10 Joe and Vito use a special custom version with a 23 round magazine.
The Main Submachine Gun of gangsters.The Thompson M1928 commonly seen with a 50 round drum. The gameplay video shows it being almost uncontrollable, this is probably because the game has no lock on system. Whenever your in the Drivers seat and your chasing someone, Joe will use this weapon until it runs out. When Henry is shot in the leg, Joe uses a Thompson to kill the man with Vito. Recently in a new trailer, the original thompson was seen. The 1928 Thompson in this game is a "franken-gun" of sort, having some features of both the 1928/A1 and the M1/A1 models. The gun in Mafia II has the M1 style stock, the M1 style bolt, the receiver of the M1, and although this is not the case with either model, it fires from a closed bolt
Since the M1 was still in use by the military in 1951 it would have been something you wouldn't find in an random gunshop. Only veterans that sneaked the rifle back home would have had them and even then it was difficult to pull off due to the size of the rifle
Seen used in a gameplay video.
A Remington 870 is seen on the table in the gun shop screen. Used by Vito, gangsters, cops, and random shop owners. An anachronism, the 870 wasn't produced until 1950 which makes it incorrect to have for the first five missions. The raised rib was also a feature that wouldn't be added till latter