A snap mechanism is disclosed for use in double-barrelled shot-guns having two hammers and a single trigger. The mechanism utilizes a sled-like selector means mounted on and displaceable transversely on an oscillating connector that connects the trigger with a rocker. The selector interacts with the snap rods of the hammers for selectively releasing or detaching first one hammer and then the other, or viceversa, depending on which of the two barrels is to be used in the firing of the cartridge.