I am building my own PCP home made gun from parts (paintball tank+ plumbing parts) but I have one big question at the end and I need one of you experts to help me out cos my physics level is equal to zero and I can't solve this.

My setup:
Paintball tank (pressure 3000 PSI) ---> regulator (regulates the air going out of the tank at 850 PSI - classic pressure for pinball guns) ---> sphere valve ---> "loading cylinder " (dunno how to call it, but it is a piece of pipe that the regulator will fill with 850 PSI pressurized air) ---> trigger valve ---> 13 mm inner-diameter barrel (50 cm or 60 cm long i haven't decided).
That s the whole project.
Now my big question is:
- if i decided to try and shoot a 12.5 mm carbon steel bearing ball as ammo, what volume (cc) should my "loading cylinder" (as i called it) have in order to be able to get a decent performance out of this PCP setup ?
I want to thank you in advance to any of you knowledgeable people that can solve my physics problem! Cos I think that is pretty much what this resumes to. Looking forward to your answers!
