A single trigger sequential firing mechanism for a double barrel firearm fires the two barrels or chambers successively upon successive trigger actuation, without need for a separate selector switch. The firing pins are driven by linear motion of corresponding pistons or plungers, rather than by arcuate motion of pivoting hammers. The present invention is adaptable to various double barrel gun configurations, but is particularly adaptable for use with over and under double barrel shotguns. Three different embodiments are disclosed, with each having identical parts and components with the exception of the upper portion of the trigger body. The first embodiment fires the two barrels successively upon successive pulls of the trigger. The second embodiment fires the two barrels after a first trigger pull and a subsequent release of the trigger. Finally, a third embodiment fires the two barrels upon successive trigger releases, after the trigger has been pulled.