CO2 guns are pressure vessels just like 4 oz paintball tanks and 20 lb bulk CO2 tanks. Paintball and bulk tanks must be pressure tested periodically, not because they deteriorate with time, but because every refill and exposure to high temperature stresses their components. A time period was chosen for retesting because in the pre computer, pre internet days there was no realistic way to keep track of refills and exposure, so a time was chosen by agreement.
Why are CO2 guns more reliable with repeated fillings when larger vessels are less reliable with repeated fillings. Has anyone ever tested this. A reasonable test would require a statistically significant number of guns. Let's say one hundred to describe the test. Fifty guns would be filled and refilled one hundred times each, and then the guns examined for wear. Fifty guns would be equipped with a pressure gauge, filled, and then allowed to sit in storage, occasionally exposed to 120 degree temperature, until they lost all pressure. These guns would be examined as the first batch. Then results would be compared. This portion of the test may take a long time. My bulk fill Crosman 111 resealed by Beeman in the early 1990's is still holding its original charge.
I propose four reasons manufacturers recommend discharging the gun after every use.
1. and most probable. Sell more C02 cartridges.
2. insurance and liability. Should a person, child or adult point a charged CO2 gun at themselves and pull the trigger they could receive a nasty burn from the cold gas.
3. Cut down on warrantee claims because users would nor realize their guns have a slow leak.
4. More long term business for the repair department.
I reviewed the Umarex manuals for the 12 gram charged Fusion and 88 gram charged Beretta Storm rifle. The Fusion manual recommended removing the CO2 cartridge after each use. The Storm manual did not. Does an 88 gram cartridge do less harm to a gun than a 12 gram cartridge. Unlikely. Umarex probably figures they would never sell another 88 gram gun if they recommended removal after each use.