"The tube " is a "lethal barrel", when it is a "component part" of a "firearm"... firearm includes air weapons. "The spring" is a "component part", and any over UK limit spring, should only be delivered to the UK to the person authorised to have it on his FAC. Without such FAC the person is not allowed to own, posses, buy or sell a component part of a firearm... firearm includes air guns. And just suppose the spring falls into the hands of the owner of an airgun with a barrel less than twelve inches, and the spring is fitted, giving power over the UK limit, would you imagine that the rifle would need a FAC?.......No it wouldn't. It jumps straight to prohibited weapon, you go to jail. Get a grip, this is serious stuff.
Pick up your gun, shove a bullet up the spout
It's the Major Dennis Bloodnok Rock'n' Roll Call Tango