T-Shirt Cannon

[asid61]

I would avoid a "custom pressure block" and just go for 100psi (or whatever you're running at) metal pressure fittings. Your barrel should also be thin-wall metal rather than thick-wall plastic.
Scuba tanks for storage sounds pretty standard from what I've seen here; just make sure you run a regulator to the output.

What is the small cylinder in the back of the barrel for?
I really like your design, it looks very neat. You could remove the twist axis if you wanted to to add stiffness (just turn the robot instead) but your way could be more interesting.