Harry, I think it a catch 22, the fact the calibre is obsolete is one thing but the fact it will fit into a .303 chamber is another, myself I'd class it as an obsolete conversion unit but its what your local force class it as.......

In recent times Section 58 firearms have had a bad press, obvious for reasons I won't go into but, an obsolete calibre firearm is just that, an old barrel chambered in an obsolete calibre is a component from an obsolete firearm, a new made obsolete calibre barrel is a section 1 firearm part..... but an old barrel in an obsolete calibre that will fit a Section 1 firearm ........... I couldn't say.