what you do is put copper on the inside of the barrel opposite each other at the bottom glued to the inside wall, drill two holes in the barrel on each side and hook up the wires from the solenoid to each hole,when you drop the projectile with metal/copper on it, it completes the circuit activating the solenoid and firing the projectile.
would this work?have i over looked something, i just thought of it so i might of
