Paintball Cannon / Launcher -
Launchable Paintball Grenade

The concept is pretty simple. You make a betty style grenade and put it in a tube that fits inside of your barrel. Tube should have a solid bottom so the betty can't slide out. Pull the pin and gently slide it down the barrel with the red cap facing out. Launch it. It may start going off immediately in the air due to the force of the launch. If it doesn't, it will go off as soon as it hits the ground.