You've got the rifle to go with them? How many of those are around?
There's a reason why a fair number of Boyes owners have converted their rifles to .50 BMG.
don't know why anyone would do it now brass can be made from .50 bmg just needs to be resized and placed between the extractor and the ejector by hand then fire formed and neck sized so it headspaces off of the shoulder.
In most cases it would be ammunition availability. It's far easier to find .50 BMG ammo or even reload your own. When was the last time you saw .55 Boyes projectiles available? Unless you're spinning your own, you won't be finding any.