The barrel joint is the only silly one. Copper pipes are expensive so I used an other one and it was too thin for the muff so I put some tape around it

It's no big deal though since that part is not pressurised for longer periods of time. At first I was quite disappointed when I tested this cannon couse it just spat the marble out and it roled on the floor. Then I attached the valve to a drill and let it spin for a few seconds. After that the valve was much easier to open so I could then open it much faster. I did a few tests and discovered that it wasnt much of a difference between using a sprinkler valve and a ball valve. The only con is that you cant aim when you use a ball valve. Here are a few damage shots at 100psi with a marble:

Obviosly it went right through, spreading tiny pieces of plastic all over my bed

This is a hard drive disc. They are made of copper, are 1mm thick and are quite hard to bend with bare fingers. Even though the marble didn't go all the way through, it still left a nice buckle on the disc.

A coin. It's realy hard to aim, I was aiming for the middle from 2cm distance =/