Imagine a .375 piston 6" long.
The 80 pounds on a .375" diameter (0.110 square inches) would create 727 PSI
6" x .11 = .66 cu-in.
IN A CLOSED CHAMBER:
727 psi is 49.5 atmospheres Your 6.0" x .375 cylinder, if a closed tank, would have to be pressurized with 32.67 in3 of gas in order to reach 727psi. Or, to state it another way....Your 6" x.375 cylinder would have to be pressurized to 49.5 times it's empty volume to reach 727 psi.
I assume you won't be using a closed tank and will be sending pressure from the piston directly into the open barrel breach with only one stroke of the piston from the bow string. In this case I doubt you'll achieve 727 psi but....I could be wrong.