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http://en.wikipedia.org/wiki/Air_gun#Gas_Spring
Gas Spring
Some makes of air rifle (e.g. Weihrauch, Theoben) incorporate a gas spring in some models instead of a mechanical spring. Pressurized air or nitrogen is held in a chamber built into the piston, and this air is further pressurized when the gun is cocked. It is, in effect, a gas spring commonly referred to as a "gas ram" or "gas strut". Gas spring units require higher precision to build, since they require a low friction sliding seal that can withstand the high pressures when cocked. Gas spring units are available as an upgrade for some popular models such as the Weirauch HW80, the Crosman 766C American Classic air rifle and the Arowsmith 876 Magnum rifle. The advantages of the gas spring include the facility to keep the rifle cocked and ready to fire for long periods of time without harming the mechanism. Also, since there is no spring (and therefore a reduction in moving mass during firing) there is less (although some say slightly sharper) recoil. There is also an elimination of the associated problems of long-term spring fatigue and a faster "lock time" (the time between pulling the trigger and the pellet being discharged). The improvement in lock time makes for better accuracy since there is less time for the gun to move off target. Finally, gas springs are practically maintenance free and last longer than conventional metal springs (research test ch1208); they are an order of magnitude more expensive when they do need replacement.
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