Thanks for the replies and feedback everyone.
Peter -thanks for the information re: chamber inserts and barrel re-lining. I remember reading your barrel-lining ideas in one of the other forums.
Vincent - thanks for the feedback. I'm pretty envious of IMA's new Mk 1 barrels for $100 USD but I can't import them unfortunately.
I can get new barrels made by an armorer who has done them before - they are about $2,200 each though, without fittings.
My idea was to tap the hole and insert a threaded plug, with the bottom of the plug contoured to fit the chamber - then weld in place.
I'll put up a drawing shortly.
Cheers,
Richard