I believe you will get an answer on the correct legal approach to this issue on the site below
http://www.cybershooters.org/phpBB2/index....25f3a121072562d
at a guess, probably a case of ensuring humane killer is on FAC, purchase .410 on SGC, chop down barrel and stock yourself and register it onto existing FAC, removing it from SGC. The Police will probably write this onto the FAC themselves - ask at above site and then approach local FLO with foreknowledge.
not a well known fact but you are legally entitled to construct your own firearm and it is easier to do this from scratch ra