(1) Actual spring or ram force to compress can be found by the formula and force required upon the lever (barrel or cocking lever); positiohttp://www.enchantedlearning.com/physics/machines/Levers.shtml;
(2) 1:16 twist the most common rate used;
(3) Volume of air in a piston gun is only a portion of how the system works. Piston powered guns generate large amounts of heat. The heat causes the air to expand, often dieseling is the source of powering the pellet but, in a properly tuned and lubed gun, the heated expanded air acts alone. Refer to "Stoichiometry".
Before you jump right on in, and get hurt possibly, be sure to ask any and all questions when a bump comes along.
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