Thanks for the replies everyone, I was hoping to have an invisible sleeve job done rather than inserts as the original barrels are honed out of proof and pitted, plus shortened so are just nice looking scrap tubes now.
I will do some measuring and consider if it's worth looking into.
Toby,I hadn't even thought about the fore end and see your point with the centres and presumably point of convergence.
The gun is a lovely old BLNE from Griffiths of Manchester with some really nice features and if it could be saved it'd end up living in my cabinet. I realise it would not be an economical fix but sometimes things catch your eye and you carry on regardless. I imagined the small action built for a B.P. gun may not be strong enough for nitro in the same bore and that's what started me thinking about going down a bore size. If it's not possible it'll end up as a wall hanger, it's far too pretty to scrap.
