Reply To: A theoretical exploration of physical possibilities, AKA Let's Make A Monster
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I would say .45 for caliber (large enough for deer in most localities, while saving .05 or so of an inch in bore diameter). I have heard that the maximum barrel length for most spring-type rifles to maintain accuracy is 24 inches with the ideal being 12; I would err to 18, so as to provide more leverage for cocking while maintaining a reasonable bore volume. Beyond 24 inches, the compressed air behind the slug loses compression and the projectile begins to drag in the barrel.
A thought occurs to me; perhaps, in order to calculate the required dimensions of the compression piston, we may need some input from the big-bore PCP cognoscenti. Namely, in large-bore air rifles (the Texan being an excellent example) how much actual air is displaced during a full-power firing cycle? How much volume at what pressure?
Most figures list the fill pressure at 3000 PSI, but does that translate to it releasing a charge of air at 3000 PSI for one shot? I suppose the volume of air needed could be calculated, as I_Like_Irons suggested, by the internal volume of the barrel. The calculation of this (18 x 0.454) gives us an internal barrel volume of roughly 8.2 square inches. Therefore, a cylinder diameter needed to compress that volume to a pressure of 3000 psi can be reverse-calculated, assuming a standard 21″ piston stroke length. Said diameter would need to be… uhhh… I’m not good with geometric calculations. Tag out?