I can't answer that simply, Keith, and I can't profess to be an expert. So with those caveats: probably yes, but there's so many variables: what's the throat like? If tapered, larger (lead) bullets can be used, and they can swage into the bore proper. BP or smokeless? It's generally accepted that BP will kick a slightly smaller diameter lead bullet in the bum and bump it up to fill the grooves. Patched or grease grooved is another variable. Alloy - hard v soft...
Sometimes you've just got to - pardon the pun - bite the bullet, make some arbitrary choices, and just throw some lead down range and see what it does, and make some educated guesses based on what evidence can be gleaned about why it's happening the way it is, then tweak a variable here and there. We can be confident it's some sort of (nominal) .500 Express, but we'd need to know the chambering, so we can make an educated guess about what powder charge and bullet weight it was regulated for. Back in the day, it may have had its own mould, or may have just been fed generic off-the-shelf cartridges of whatever diameter was in use.
Graeme Wright's "Shooting the British Double Rifle" is a great starting point; Paul Matthews' "The Paper Jacket" another.
