Cox’s Bolt Gun.
This formidable piece of salvage gear could shoot a bolt clean through sheet steel one inch thick using an explosive charge! The gun itself weighs 16 kg (36 pounds) and was usually lowered to the diver on a rope or chain.
Before it was invented divers would have to spend hours drilling, screwing and/or welding fixings onto steel and wood underwater. The Cox’s Bolt Gun reduced the work time from days to hours.
It could be used above or below water without recoil, flash or report (according to the book, Deep Diving and Submarine Operations book).
One type of bolt was hollow with a detachable point. It could be used to deliver air to stricken submarines. The bolt could be fired through a submarine pressure hull, an air-line connected to the back of the bolt and then, at a signal, a submariner could remove the point from the inside of the submarine and fresh air could be pumped down to the submerged vessel.