This is something I made because I wanted to be able to fire something from a robot. On the inside of the end cap there are two insulated wire holders that are used to hold a small peice of nichrome wire.
To load it you first unscrew the end cap and put a small peice of nichrome wire in the wire holders. Then you fill the end cap halfway with black powder and put what you want to launch in the barrel. When you apply sufficient power (depending on the size of the nichrome wire) the nichrome wire will heat up and fire it.