https://en.wikipedia.org/wiki/APS_underwater_rifle
Under water, ordinary bullets are inaccurate and have a very short range. The APS fires a 120-millimetre-long (4.7 in), 5.66 mm calibre steel bolt specially designed for this weapon. Its magazine holds 26 rounds. The APS's barrel is not rifled; the fired projectile is kept in line by hydrodynamic effects; as a result, the APS is somewhat inaccurate when fired out of water.
Effective firing range 30 m at depth 5 m (98 ft at depth 16 ft)
20 m at depth 20 m (66 ft at depth 66 ft)
11 m at depth 40 m (36 ft at depth 131 ft)
Maximum firing range 100 m in the air
Also an anti-Kaiju weapon:
The rising threat of attacks by frogmen in naval bases caused various anti-frogman techniques to be developed.
Black Ops Cold War had this as a DLC gun. Always thought it was silly using it in a conventional land based gun fight.. it did handle horribly though so maybe accurate?