Underwater guns are still being developed and marketed.  Here&#x27;s a description of a modern supercavitating Norwegian one: <a href="http:&#x2F;&#x2F;www.hisutton.com&#x2F;Special%20Forces%20diver%20technoloy%20-%20DSG%20Supercavitating%20Ammunition.html" rel="nofollow">http:&#x2F;&#x2F;www.hisutton.com&#x2F;Special%20Forces%20diver%20technoloy...</a><p>(The book that website promotes is a good buy if you are into this kind of thing.)