I think the Russians have had a supercavitating torpedo for some time.
Hard to imagine reducing that tech to a rifle bullet.
Correct. The Russians have had this to some degree for several years. Who knows their degree of success.
The torpedoes are referring to how their propellor’s edge cuts through the water.
I’m guessing the bullets supercavitate because their shape allows them to apply higher pressure on a smaller point of attack which then boils off the water in front of them allowing for reduced drag.