The English rook rifles are pricey these days, and if they need substantial gunsmithing to put back in order, as many do, particularly if they were converted to .410 shotguns, the gunsmith bill is not for the faint of heart or wallet. John Taylor did several barrels to fit my pre-war H&R Garden Gun actions, and I have one "real" English rook rifle, which was relined and rechambered from a .255 to .32 S&W Long, then the outside restored by Lucas Geiger.
The old soft steel actions, which include typical American break-open shotguns, should not be converted to high-pressure cartridges, because even after bushing the firing pin, with heavy loads as in the .327 Federal or .357 Magnum the actions will shoot loose. The .32 S&W Long, .32 H&R Magnum and .38 Special +P are all viable choices. My recent .30 Carbine barrel will be limited mostly to.32-20 equivalent handloads.
73 de KE4SKY In Home Mix We Trust From the Home of Ed's Red in "Almost Heaven" West Virginia