[KenWittlief]

If this is something you are going to use at public events, and I assume pointing at people in the audience, then my inclination is to remember that 'entertainment' is 95% illusion and deception!

I would make something that is spring loaded: pull the spring back, put the tee-shirt (projectile) in the barrel and release the spring. Two moving parts, very easy to ensure nothing can break off and become an unintended projectile.

But, entertainment! you can make it look like its air powered, stream powered, or a quantum rail gun, and have pressure tanks and gauges and lights and valves and a night vision scope, even have it shoot out CO2 vapors when it fires, and only you and the school board (and their lawyers) will know its all smoke and mirrors, with a small spring inside doing all the actual launching. :^)

In fact, with this approach you will probabally end up with something more impressive looking than if it really were air powered.