A bangstick is a variety of underwater speargun used against sharks.
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Unsure what you are looking for. The term "Powerhead" is used with specialized firearms used against sharks by divers. Sometimes known as a "bangstick", the powerhead is a single shot firearm, mounted on a long stick. When shoved hard against a shark, it fires a single cartridge.
Maybe. Most will fire underwater- the cartridge contains powder that does not need air to burn. Some guns may even fire safely underwater- not all can (water blocks barrel). However, the resistance of the water limits the distance the bullet will travel to a few ft. The military of several countries have experimented with building an "underwater" gun" that had rather iffy success. There IS one specialized type of gun used underwater- called a "Bangstick". This is a long metal rod with one cartridge in a holder at the tip. Used as a last defense against sharks, when shoved hard against a shark, the cartridge fires. The bullet hits the shark, but more damage is done by the gasses generated by the cartridge firing, being blown into the shark.