Instructables user Professor Hognutz has a step-by-step instruction on how to make a lip balm gun:
This instructable will show you how to construct a less-than-lethal, easily concealed, personal defense weapon. The design's only drawbacks are that it is a single shot weapon and it has the killing power of a marshmallow dropped from the height of two feet. You might be suprised by how far it shoots though. You'll lose the projectile before you know it!