In the video, Rober walks through exactly how he fabricated this beast, and even chats with the inventor of the original Super Soaker. It's revealed that the gun's facade matches that of traditional Soakers, but underneath it's a whole different story. Rather than harness pressure via a hand pump, it gets its water pressure from a tall nitrogen tank hidden in the handle, which shoots the water with just a little more oomph than you're probably used to. OK, a lot more oomph, like 2,400 psi (pounds per square inch) and 243 miles per hour kind of oomph (for reference, the original Super Soaker shot at 40 psi, and fire truck hoses shoot at 300 psi). Needless to say, this thing is dangerous as hell, capable not only of shattering glass and slicing a watermelon cleanly in half, but ripping through a human body if you're not careful.
If you're up for the challenge of building one of these yourselves, Rober has a link to the CAD specs they used, but he also warns that you better know what the hell you're doing and take extreme precaution because "I don't want to have to attend your funeral because I don't like to travel."
Consider yourself warned.