The term 'MOBSTER's Gun' can be summarized by two crossword solutions: ROSCOE (6 letters) and GAT (3 letters). ROSCOE is a slang term for a gun, usually a revolver, that was popularized in the 1920s and 1930s. The term is derived from the name of a character in the comic strip 'Barney Google and Snuffy Smith'. The term 'GAT' is also a slang term for a gun, usually a semi-automatic pistol. It is derived from the phrase 'Gatling gun', which was a type of machine gun invented in the late 19th century. Both terms are still used today to refer to any type of gun.