Legislation on the carriage of dangerous goods changed a few years back and brought in restrictions on ‘pressure vessels’ as opposed to ‘pressurised’
This means that any ‘normal’ carrier will show them in their ‘restricted’ or ‘prohibited’ goods lists (probably prohibited) and then you can only use a specialist carrier, or as retailers will (or ought to) use their contracted carrier on specific runs etc
Blame the people who used to lie on their declarations and had air cylinders found on routes that used air Mail (a lot more than you may think)
If you manage to find one, then ideally the cylinders regulator should be removed (rendering it impossible to pressurise - but doesn’t stop it being a ‘pressure vessel’
A lesser measure is to remove the full nipple and/or the high burst disk - but anyone running the package through an X-ray won’t notice that - they will still see a cylinder with a fitted regulator
(***** of course it must be fully depressurised before removing any parts *****)
