A concrete nail gun is a very simple tool consisting of a hollow metal barrel and a firing pin. Actual gunpowder from a modified .22-caliber shell propels specially designed nails through the wood and into the masonry. Either with a hammer blow or a trigger pull, a firing pin strikes the back of the shell, setting off a controlled explosion safely contained within the tool. Gas from the detonation escapes through the barrel and drives a nail that has been placed there.