Morris Tubes

[Buccaneer]

First things first. A BARREL per-se is a firearm in its own right, as defined in law as a thing capable....... blah blah blah and so on. Whether it fits inside another rifle or fits into a 25 pounder or MOBAT or WOMBAT (to act as a 'bang maker' ) or whatever shell case for whatever reason is purely academic. If the barrel falls into the category of 'obsolete calibre' then by default AND definition it is, of itself an obsolete calibre firearm in law. You can't have a 'half' a firearm......... It is or in this case, it isn't one requiring a certificate. You - or the vendor - can make it as simple or difficult as you like

Peter is quite correct the item in question can only chamber a recognised obsolete cartridge and therefore can be owned under section 58(2) the fact that it will fit into a .303 barrel is of no consequence at all. If however it is intended for use then a variation would be required AND it would need to be sent to the proof house to be proof tested before use, this requirement is ALWAYS pointed out when selling any section 58(2) firearm.
To prove a point I own a .577 450 Martini Henry rifle, I also own the 297 / 230 Morris Tube for the same rifle, they are both section 58(2) however if I wanted to shoot them they would both need to go onto an FAC AND be proofed if either or both of the items were to be used on public / private ranges, what people do in there own back yards is down to them.