How big should the chamber be?

I planned on using a 12"x3" tube for my pressure chamber? Is this too big? or too small? What could i use to pressurize it to 300psi if not helium?

Yes preferably shoulder mounted but it might be too heavy for that(air supply can be separate)

Well I made a PVC cannon recently and it gets around 50 yards with 115 psi and a 3" barrel. So I wanted to go bigger 150 yards isn't necessary 100 is what I'd prefer. I know that commercial cannons can exceed 400 feet so I thought I might be able to match that or at least come close. I was planning ...

I plan on building a t-shirt cannon with a 3" barrel and would like to achieve distances of around 150 yards if thats possible. I plan on making it out of aluminum but wasn't sure if steel would be necessary, I was using aluminum to keep the weight down. Are there better options than aluminum or ...