The only 'alarm' guns that I have seen, used a short section of tube which took a 12 bore blank, there was then a spring loaded firing pin held back with a further pin, this in turn was fastened to the trip wire. The whole device was then designed to be fastened to a fence post or tied to a tree. On the device in question, the tube to take the 12 bore round, had been welded on in place of the blank container. It was later tested and proved to fire a live round, but not without damage to the device. Remember these people were converting 'de-activated' guns and producing 'replicas' to fire live rounds. They are all presently 'inside'.