Looking at that action, I cannot help but think this once used to be a supine match rifle chambered in something like .461 Gibbs. How about it, to recreate something like that?
FWIW, Ueli Eichelberger in Switzerland still will make you a (early pattern,with gain twist if you want, but not segmental rifling) Metford-rifled barrel, and good work too... I have one of his barrels on my #2 Musket, and it really is a quality barrel, very accurate and cleans easily. When compared to my .45 Snover with (yes!) Metford segmental rifling, I do have to say it is better. But the Snover is much better with PP bullets though.