Thank you Wayne, John and others for your thoughts and suggestions.
I have been in contact with McDonalds Wayne and as soon as I can get to Melbourne I will go and have a look at the u/o 9.3's. I think the point was missed that my idea was never to just re-chamber the Baikal, but to utilise the .458 barrels with their regulating system, and have them adapted by a good gunsmith to fit the Fausti 12G sxs action. I thought that would work out easier than going through the whole saga of starting with blank barrels. In any case I have given the idea away since hearing the weakness with the Bailals was not in the action, but in the barrel, with the 45/70.

Thanks, Ian.