A revolver was a type of gun.
In River Song's World, Holy Roman Emperor Winston Churchill carried a revolver while speaking with his soothsayer, the Eleventh Doctor, at Buckingham Palace. The Doctor noted this after feeling the sensation of having been running while having no recollection of doing so, and Churchill justified himself by claiming that the Doctor was "dangerous company". Later, Churchill found that he had fired his revolver when he smelt the gunsmoke, with the Doctor soon realising that they had both been engaged in a confrontation with the memory-proof Silents. (TV: The Wedding of River Song)