No, that is a underwater round, not an underwater gun. Most guns can be used underwater, but range is limited. The bullet in that round extends range from normal rifles. It has the added advantage of being able to be used in air as well (for "over the beach" assaults).
But you are right, they are still developing them. Russia unveiled a new one a couple of years ago (I don't have a link on hand)
(The book that website promotes is a good buy if you are into this kind of thing.)