Thanks to all of you for your ideas. I think that I have determined that the .668 muzzle will be fine for a re-bore. That leaves me with choosing a bullet so that I can get the throating and rifling twist right. The bullet will be partly determined by the overall length of the loaded round which will have to feed through a Lee-Enfield magazine. At the moment I'm investigating the Lyman378674 mould at 335 gr or the 264 gr #375449.