The gas in a 12 gr. CO2 cartridge, when expanded at room temperature, will occupy a volume of 400 cu.in., so at 1 FPE/CI (which is good efficiency) that would be a total of 400 FPE.... as in 20 shots at 20 FPE would be very good performance.... I read somewhere that in theory there is over 6000 FPE inside a CO2 cartridge, but I don't believe it.... If you are talking about using all the CO2 in just ONE shot, however, you will never approach the 400 FPE level.... not even close.... Bob
What caliber do you plan on the rifle to be?