CMWill,
You have a true gem there, a best quality gun in anyone's book. It appears that it was originally chambered for one of the now-obsolete 8 mm's like the 8 x 57R/360. It was rechambered in an English-speaking country, not England, as it would have required reproof and I see no English proof marks. My guess is that it was done in the USA. The bore diameter is clearly stamped .317 Dia. as is the caliber, 8 mm-.30-40 Krag. Also the name of the company that did the work, looks like I O Stone Co. That means that it uses a .30-40 Krag case, necked up to accept a .318 bullet. Be careful there, if you are not aware that modern 8 mm bullets are .323 diameter, not .318. If this gun was originally designed for jacketed bullets, it would normally be stamped StMG, for StahlMantellGesosch (steel-jacketed bullets). I don't see that stamp so assume that it may have used lead bullets originally. I would guess that you'd have best luck trying to regulate this rifle using 180 grain bullets, sized .317", and starting at around 1700 fps.
One last thing that I'm surprised nobody has pointed out. Back when this rifle was built, Europeans didn't put crowns on things being built for Joe-Schmo's like us. That symbol was reserved for royalty and the nobility. That gold crown and gold initials, inlaid into the horn triggerguard, suggest to me that this rifle was built for somebody pretty important. One of our European friends may wish to comment on that. I'm certainly no expert but that's my take on it.