The question you raise is something that has worried me for some time.
Compressed air bottles or cylinders used in conjunction with pre-charged air rifles must, by law, be tested visually, internally and externally, and subjected to a pressure test 1½ times their working pressure every five years.
This does not apply to vessels of less than 1L capacity. No test standardisation has yet been established, nor a method of valve removal and internal examination of these smaller items, nor a method of regular testing of integral air chambers or reservoirs.
Guns powered by compressed air are excluded from the Gunbarrel Proof Act, but the pressure contained in the case of rifles charged at 300 bar, if subjected to a hydrostatic test pressure of 450 bar, would be only slightly lower than the service pressure of a 2½in 12-bore shotgun cartridge.
If pre-charged air rifles are filled using impure air, then internal corrosion can occur in some cylinders or reservoirs made from ferrous-based materials.
I think legislation to ensure public safety is well overdue.
One comforting fact is most of the reputable manufacturers of pre-charged airguns now incorporate a ‘fail-safe’ device in their designs to prevent high-pressure damage.