Unbeknown to me, China has been developing a line of underwater guns. Their designs are based on Russia’s underwater guns but are chambered in a new cartridge that appears to use 5.8x42mm (Chinese DBP87) brass.
The Chinese version of the Russian SPP-1 three barrels in a triangular configuration, rather than the SPP’s four barrel setup.
More info at China Defense.
[ Many thanks to Sven (Defense and Freedom) for emailing me the the info. ]