My neighbor's son knows we play paintball and that my son mods nerf guns for his friends. He showed up with one of these. It's pretty sad for the effort that went in to it.
The 'balls' are actually solid gel, slimy like most water absorbent materials you find in cable packing or diapers. The balls tend to be different sizes, worse than paintballs, but generally around 50 cal.
The gun mechanism is all in the pull rod in the back. You pull it back and let it go, like an airow gun. No trigger. There is a "carrier" or plug that slides up and down the barrel that gets sucked behind the ball-drop and then forces the ball out of the barrel from the air pressure. It's more like a restrained catapult.
Generally, it has direct range of about 3 ft and lobs about 10-20 ft, depending on luck and technique.
The primary mode of failure is for the kid to put too many pellets in the 'magazine' and grow them in water. The pellets are then too stuffed to move out of the magazine, and you have to wait about a week to let them shrink enough to come out. Other than that, they are mostly harmless (i.e. not very entertaining).
There, my first MCB review.