A Peacemaker was a type of pistol favoured by residents of the western United States during the 19th century. According to the Tenth Doctor it was a "single-action .45 calibre pistol" manufactured by Colt in 1873 and "the pistol behind a million gunfights, range wars and shootouts." (PROSE: Peacemaker)
Behind the scenes
The Doctor's synopsis of the gun is essentially true of the real world Peacemaker in every detail.