I have a double rifle in 318 Westley Richards. The barrel diamensions are. 5 groove, .325 lands, .329 grooves. This rifle was made by Thomas Bland in 1911. It shoots .338 bullets fantastically. The neck must be thinned for the larger dia. bullet. Many gun makers at the turn of the century(Victorian) made their guns larger in the grooves, for fouling. They were still use to black powder.
Always slug the barrel before you shoot. You may have a pleasent suprise!