65.3cc's by my maths
,
As I said above, in a PCP the energy that you have put in to the springers spring when you compress it cocking the rifle for each shot, does not exist
so that stored effort/energy has to come from extra compressed air.
For a given pellet in a given bore & barrel length, the energy (from the compressed air) required to reach a specified muzzle velocity will be exactly the same ( vol A), so the shot is equally efficient.
BUT to replace the energy from the spring you need another 150%ish of vol A (vol B).
All vol B does is push vol A in to the breach
it then exits the muzzle as wasted air once vol A has shot the pellet out.
In a gas ram vol B is contained inside the ram, but You again have to impart the energy in to the gas by physically cocking the rifle to compress the gas for each shot.