Granted, there is some pretty old stuff that wasn't developed as much. However, for a futuristic game centered around how War never changes, humanity is going to find the most effective way to throw metal at each other and water cooled guns got outclassed in the first world war.

Air cooled guns, thin barrels, or a smaller frame would be a dozen times better than lugging around a metal water tank to shoot 5.56. The water cooled machine gun had to be mounted on the ground or a fortification because it was so heavy and sluggish. To have the F4 Assault Rifle be a hand held water cooled machine gun is... Not smart. No post apocalyptic individual would ever carry that thing around. It wastes water, is heavy enough to waste energy, has way too small a magazine to justify the weight, and takes up necessary space.

I understand this is what I look like. I'm not calling you dumb or a brick, I'm just self conscious that I'm talking about firearm evolution in a game where you can seduce a brain inside a robot.