I'm pretty sure that's the "3D Printer Gun" that Sumireko use in ULiL, not a water gun. But I can't really say that it is made using 3D printer in this picture. I don't know what should I tag it as, someone help.

Considering it's on runners, that's probably the Plastic Model version. After all, it's hard to discern building material in illustrations without text indicating what they are. Drawing it with used plastic model runners is easier to tell the gun's made of plastic than showing it with a 3D printer.

Edit: Runners mean Plastic Injection. It's gonna be a hell lot more expensive than a 3D-printed one, since you'd have to have access to an injection die. Unless they come in kit form and it's mass-produced like Gunpla, that is.

Oh man. Now I really want someone to make a gundam style model kit for this and sell it.

"Once the 3D Printer Gun is mass produced..."

... i wonder if a 3D printed Tommy gun could be possible..?