Step 12: Try it out
Get a bucket of water, push the inner pipe all the way in, put the end cap under water, pull the inner pipe back until you can just see an o-ring, quickly tip the end cap skyward to keep leakage to a minimum, place the end of the inner pipe on the ground, tilt in the direction you want to fire, pull downwards on the outer pipe to fire.
Bonus fun: if you pull the inner pipe back and seat an undrilled end cap, you can launch the end cap with pneumatic pressure, generating an impressive bang in the process. Make sure it's not aimed at anything or anyone. Also make sure you're not inside your garage when you discover how cool this is like, uh, someone I know was: a garage door makes a resounding boom when you nail it with an end cap!