a coach gun is a modern term, coined by gun collectors, for a double-barreled shotgun, generally with barrels from 18" to 24" in length placed side-by-side. these weapons were known as "cut-down shotguns" or "messenger's guns" from the use of such shotguns on stagecoaches by shotgun messengers in the american wild west. they came in 10 and 12 gauge blackpowder.