In the film, Mark David Chapman (Jared Leto) carries and uses a Smith & Wesson Model 36. The revolver that was used by Chapman to shoot John Lennon in real life was a Charter Arms Undercover, but the Smith & Wesson Model 36 bares a close resemblance to a Charter Arms, and was probably used due to its availability.
Trivia
In the film, an actor named Mark Chapman (middle name not David) plays John Lennon.