JWR,
Reader M.D.W. wrote a very informative article, as far as he went.  The newest technology in the air rifle race is the nitrogen piston, replacing the metal spring on the break action guns, both in rifle and pistol format.  The nitrogen piston, or nitro piston as it is known, uses the technology of the auto shock absorber.  When was the last time that anybody heard of one of those failing?  The nitro piston can be purchased as a spare and stored indefinitely on the shelf with no special attention.  The major advantage of the nitro piston is no spring bounce after the piston bottoms.   There will be no further vibration from the piston section, unlike the spring that still moves slightly back and forth after the piston bottoms.  These rifles can hold a Quarter-size group at 25 yards.