I've repaired a few Airguns in my time and it is obvious to me that the storage of guns under pressure should be done at relatively low pressure ranges. For an Airgun like a Career or Sumatra I've observed that the orings tend to want to extrude out the gap between tube plug and tube, valve body and tube, gauge oring and gauge body.
This is partly due to the relatively soft orings the Koreans utilize but the storage at full pressure certainly exaserbates the issue.
The mainseal/exhaust valve can get overcompressed over time and when that happens the gun will loose power. Three factors are affected when seal overcompression occurs. First you loose stroke because the same lift of the Exhaust valve won't give as much flow because with the seal overlapping the seat the gap is reduced for the air to flow through. Second the stem is closer to the hammer so the hammer looses momentum due to the fact it has less run(acceleration time) at the stem before it hits. Third the seal to seat contact area is greater and wider than orignally and this translates into more of the hammer momentum being used to lift the seal off the seat and thus less remaining to get the lift you need to shoot hard.
So the question becomes how much pressure should you leave in them? I think 100-120 Bar is appropriate to avoid the pitfalls discussed. For the same reason I think it is appropriate to leave older CO2 guns with what is left of a CO2 rather than a fresh one.
Heat can also be a contributing factor, especially with CO2 guns, so if the gun is going to be stored in an environment where there is extreme heat like a hot trunk or car left in direct sunlight it wouldn't be a bad idea to reduce the pressure further.
We use relatively hard seals to counter this but the more the heat and the more the pressure the more distortion we see in the seals.



"NO GUNS WOULD BE A RIOT"

Later

Tim