Projectile Launcher

I regularly look on the Instructables website (esp. the tech section) for various hacks and projects that I can pursue. One such Instructable was the butane-powered "gun" (more like miniature cannon -- it's not very strong) made from a Crayola marker and a Bic pen. And a lighter. So several weeks ago, I built it. It works. Barely. It works exceedingly well with the right projectile (rubber o-ring thingy with a screw through the middle). Pencil erasers are especially good projectiles -- they fit perfectly in the Bic pen barrel. However, it is not efficient nor is it strong enough to be of any practical use other than proof-of-concept. I'll post pictures and link to the Instructable later.