It went unnoticed at The Firearm Blog but, back in late 2009, Russian arms manufacturer Tula / KPB announced an underwater ammunition system for the Tula A-91 assault rifle. A modified 5.45x39mm round appears to be loaded with a long thin sabot bullet. Unlike the ASP round, also based on the 5.45mm cartridge, this newer round does not need a special rifle with an oversized magazine.
A video at TVTula.ru shows the rifle being fired underwater.
UPDATE: The ADS is the amphibious variant of the A-91.
[ Many thanks to Sergey for emailing me the link. ]Related