The SPP-1M Underwater Pistol was a specialized handgun made in the Soviet Union specifically for use by Soviet combat divers. They were made during the late 1960s and later accepted for use in 1971. Its ammunition consisted of small steel darts, due to firing bullets underwater being extremely inaccurate.
Behind the scenes
The SPP-1M was originally supposed to appear in Metal Gear Solid 2: Sons of Liberty as a weapon to be used at some point of the game (most likely during the Plant Chapter due to underwater travel playing a key role in the chapter). However, it ended up cut from the final version. Players can find data on the gun in the Trial Edition demo for the game, and can even use it via modding. However, the gun had not been rendered, so it only depicts Snake holding an invisible weapon and bullets being fired when in use.