SONG CREDITS AT THE ENDMOAR INFO-Automatic Revolvers
Vash's gun is based off the Mateba [link]
(which was also in Ghost in the Shell) The reason the barrel is on the bottom is because of the recoil action to cock the hammer. This was intended to ease the trigger pull required for the usual double action revolvers.-Hammerless semi-automatics
Guns like the Glock use strikers: [link]-Single and double action with semi-automatics
Semi-automatics are often double action, but some, like the Colt M1911: [link]
are single action, but only after the first shot. The recoil action takes care of the rest.