From Sega Retro
Monster Gun is a 1972 electro-mechanical shooting arcade game created by Sega. The object of the game is to use the gun to fire plastic pellets at black-lit "monster" targets on the playfield. There are swing-away targets that run in a circle, flop-over targets that move back and forth as well as hole targets that open and close at random. Play time is 70 seconds or 35 shots, whichever comes first.
The power behind the gun is generated by a shop vac type system. The "suck" on the shop vac sucks the balls from the back of the game, and brings them to a loading mechanism under the shop vac's guts. Then, the "blow" on the shop vac propels the balls through the gun. The only catch is the shop vac and all the associated tubing needs to be clean and have tight fittings (to avoid leaks). The inside of the game needs to be kept sparkling clean—otherwise, the shop vac will suck all the dirt into the ball tubing, and slow the balls. It's not the "suck" portion of the shop vac that propels the ball, so leaks in the ball feed tubing, around the black cardboard shell, and around the black cardboard shell feed to the copper tubing really don't matter (there just needs to be enough "suck" get feed the balls). It's the shop vac's exhaust, the "blow", which pushes the balls. The only tubing here is the flexible tube from the copper to the gun. Note the ball feed tubing seems to be the same semi-rigid tubing as used in underground lawn sprinkler systems.