Quoted from nibre:
I have a chance of buying a Terminator 3 but the back box game does not function properly. The "gun" is moving up/down but the white ball isn't shot. Is this an easy thing to fix or could it be difficult to repair? Unfortunately, the game is far away and has to be sent to me so I cannot check it out.
The RPG canon is just a coil there that launches a plastic ball. The issue could be anywhere along the circuit for that coil, but servicing it should not be too difficult (glass comes off, and you can access it pretty easily to see if it is fried (or likely just loose solder on the coil connector).