I take a cheap nipple. Drill out the flash hole to the same size as the inside of the nipple.
Take an old truck valve stem, (some of them are 6" long), take the valve out of it and cut off about 2" and set it over the nipple and silver solder it. Replace the valve. All you have to do then is to screw this in your nipple hole, and then hit it with any air compressor. Make sure that you have the muzzle pointed down in the dirt or into a blanket or something soft, otherwise you could have an uncontrolled launch.
B