Westley Richards no.2 - that would probably be no.2 Musket (500/450), given it's a match rifle. Get that beaut up and running (with black powder of course!), the no.2 is a very nice round indeed, my personal favourite BP cartridge. Easy to load for, inherently accurate - really an ideal longrange BP round that will shoot with the best of them.
BTW, I would love to know the chamber/throat configuration on that rifle, so if you're planning to make a chamber cast...
BTW, better start practising the supine position - after all, that rifle was made for it.