Mechanical Engineering

I would like to built a shooting target which is comprised of (2) targets layered, separated by small springs. Basically, you have a rear target that is 8x10 and a front target that is roughly 6x8. The two targets will be separated by springs that will allow the front target to touch the rear target with a little force. When the front target touches the rear target, I would like to know what kind of sensor I need to be notified of a hit? This sensor could indicate a hit by means of a sound, or a light and I need to know what kind of sensor, possibly 12v. thanks.