44-77 Sharps paper patched load

[Fogman01]

I am examining a Rolling Block Creedmoor original. I have it for a three day look and permission from the seller to do a chamber cast on it. It has 44 S stamped on the barrel. So I just had it chamber cast tonight at my gunsmith's. It is in fact a 44-77 2 1/4 case and chamber exactly. It takes the Shiloh 44-77 Jamison brass from Buffalo Arms that I have for my Shiloh Sharps - I put a case in the chamber and it was a good fit. Did not seem to be a headspace problem with the Shiloh brass rim being slightly thinner than an original case was. The bore is like a mirror - as nice as a new rifle's bore. Amazing. But - the bore measures .451. When the gunsmith put a .446 bullet that I load for my Shiloh 44-77, it pushed right through with virtually no resistance. Is this going to be a problem reloading with neck brass expanding too much? Can I size back down so I can use it in the Shiloh too? If the bore is .451, then what diameter bullet do I load for it? Grease groove and paper patch both. I am going to try to shoot paper patch through this original Creedmoor and shoot it at matches. Was this rifle re-bored for a .45 caliber at some point in the past? Or did the originals have that much slop? The rifle is pricey but has good wood, a 5" original vernier tang and an original spirit level globe front sight.