Some of the gunbreaker ones do have normal barrels. The Ronkan manatrigger being an easy one to see. Weather the Ronkan were the not-garleans of the first i do not know. The gunbreaker itself doesn't make use of the barrel often. the LB3 fires something out of the end to form the shield, but it might just be a magic puff as the same thing gets done on a lesser scale with Heart of Stone.
In a lore friendly way I assume the blade acts like a focus and directs the aetheric charge, or quickly dissipates it in the case of explosive hits. It can be assumed it needs reloaded often, but that is something we don't do in game for MMO reasons. Just from observation of the models at hand, i also assume the flintlock looking ones are very early designs requiring muzzle loading of charges.