The transfer port is more related to the bore diameter than the gas used.... It is to prevent damage to the pellet on loading.... You can use up to 80% of the bore, if you carefully debur the port and slightly round the front corner to prevent it working like a cheese grater as the pellet slides past it.... On a .22 cal, that is 11/64" (0.172").... However, I prefer to stay with the 75% width and make the port oblong (longer than it is wide) if I need more power than I can get with a 75% port....
You can, of course, use smaller ports to save gas, but that will also limit the FPE.... For maximum power, all the ports, from the valve throat to the barrel (and inside the chamber) need to be the full area of the bore.... The valve throat can be up to 10% larger in area (don't forget to allow for the valve stem area), and to maximize the flow you will need a retractable bolt probe or nose.... The barrel port will end up being about 80% wide and 120% long.... eg. in a .22 cal I would use about 0.17" x 0.27".... This is really only necessary for a bullet shooter, most pellets can be driven into the 900s without oblong barrel porting....
Bob
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