Answer: Samuel Colt
The question in the movie was "Who invented the revolver?", but this phrasing is more accurate. Colt apparently never claimed to have invented the revolver, although he was the one who modified it in a way that made it successful. In 1818, 17 years prior to Colt's English patent for the revolver (1835), Boston's Elisha Collier patented a flintlock revolver in England, which was primarily used by the British army. Colt, who was from nearby Hartford, Connecticut, said that the innovations in his design were inspired by Collier's basic idea -- which is part of the reason that he patented his design in England a year before patenting it the U.S. Colt was also inspired by Eli Whitney (inventor of the cotton gin and one of the innovators of mass production), who in 1798 started a Massachusetts factory focused on building rifles with interchangeable parts for the U.S. Army. Colt's revolver had continuous-fire capability and interchangeable parts, which made it both useful and affordable ... and made him extremely wealthy, although it took the revolver a while to catch on.
In the movie, Jamal's older brother Salim called himself "the man with the Colt .45" when he demanded that Jamal surrender Latika to him, after he killed Maman and then was hired by the gangster Javed.