Not technically a water gun, but a high power steam/water hose. It avoids the fuel/ammo problem of the flame thrower, as it can be hooked into a water line in the mall, and the physical prowess needed to cleave 1000 zombies with a katana. The pump/boiler required is about 1.5/1.5/3 feet, (or a little bigger) , so it's a bit large, have to have wheels on it. However it can spray a narrow spray of boiling water with 3000 to 10000 pounds of pressure, which can be used to cut cement or the sides of oil tanks. It's normally attached to a steam truck, but i'm assuming they can be detached.