After the Soviets developed the SPP-1 underwater pistol by 1971, they still feared enemy frogmen attacking their naval bases and the pistol had short range by depth of 6-18m. So an underwater version of the AK was developed at the Tula plant that became operational by 1975. This was a modified type with a massive flat-sided 26 rd magazine that fed 5.66x39mm (120mm long) darts into a non-rifled barrel. A unique gas system helped to fire the weapon which extended depth ranges out from 11m-30m, allowing the Soviet divers to attack enemy suits, armored glass, and special shielded light underwater vehicles! The weapon was functional yet heavy at 8 lbs and the slab-sided mag made weilding it hard in the water. The gun could fire rocket rounds as well that left signs of where the shots originated from! The weapon was exported to Communist satellite nations, but mainly E Germany. W Germany upon reunification shipped them back to the USSR as unwanted and gave some to the Polish Navy. Russian developed an amphibious version as the old APS was almost useless on land due to the unrifled barrel and could only fire 180 rds before the gas system failed with the air. Underwater, the APS could fire 2000 rds before wear set-in. AFAIK, this is the only underwater rifle ever developed (Note: the WW2 German rocket bullet pistol and rifle gun prototypes probably could have fired underwater with slight mods).

http://world.guns.ru/userfiles/images/assault/as69/aps.jpg