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 unless it was rebored or otherwise screwed with.  Could this rifle have been re-bored for a .45 caliber at some point in the past but left it chamber for a 44-77 2 1/4 case? Or did the originals have that much slop? The rifle is pricey but has very good wood, very good metal, a 5" original Vernier tang and an original spirit level globe front sight... plus a large but immaculate bore.

What size bullet do I use to slug the barrel? The .446 slides right through with a little resistance right at the end of the barrel. 
What I don't want is rifle that somebody screwed around with in a way that will affect shooting it. The rifle is good shape compared to others out there that go for the same or much, much more. The condition of the rifle might lead me to believe that bore is original but I can't imagine a bore out to .451 for a 44-77. If it is refreshed - it will go back to the seller. 

In his book Shooting Black Powder Buffalo Rifles, Mike Venturino mentions the .44-77 groove size being closer to a .45 caliber.

To start with I'd never slug the bore with a bullet. I use soft round muzzleloader balls, with the bore oiled. They work much easier, and wont chance jamming in the bore like a bullet can.
The bore size sounds much larger than my original Remington also marked "44S" and in .44-77 SBN. Mine is a .446" groove diameter. If your gunsmith did a chamber cast, then you likely already have a groove diameter for the first couple inches of bore, as anyone doing a chamber cast should always include 2"-3" of bore to see that and the lead. 
If it is a .451" bore, then groove is even larger, but you can mic the chamber cast to see groove diameter. Maybe you meant "groove diameter" when you posted the .451"? If it's a .451" groove, then that's the bullet size. I try to never go more than .001" larger for grease groove bullets. I don't paper patch, so can't help on that size.
I'd also check the chamber cast to see outside neck diameter of the chamber. This will tell you how large the neck needs to be, and if it will handle the bullet diameter and still chamber fixed cartridges. Sometimes if a gun is re-rifled, they open up the neck in the chamber to handle larger bullet size.
I'd agree that if the gun isn't a bargain price, I'd pass if it's been reworked. But if it was a bargain, the larger bore wouldn't bother me.

THat is the correct size for  R.E.M. Creedmoor I hab one and shot it at Raton years ago also have a Lyman mold .451 do for a grease groove bullet bout 450 gr . The 44-77 would take 90 grins with th -per patch bullet barely in the case as they did back then.

Not the same caliber, but I have a Remington Match B in .32-40 with 34" Creedmoor length barrel. Bore and chamber are original, but much too large at .3275" to chamber fixed ammo. I have to breech seat bullets to shoot the gun, and guessing it was either breech seated or muzzle loaded it's entire life.
I have a friend with the same gun, but a 30" barrel, and his bore is also the same size and can't shoot fixed ammo in it. Guessing for these Match B guns that Remington made them larger and expected people would muzzle load or breech seat them back then.

bruce moulds wrote on Jul 4^th, 2017 at 6:08pm:


It might, but I'd see no reason to, as that would be either too small, or require me to shoot paper patched bullets and BP. Neither is on my "want to do" list.

I decided to keep my RB Creedmoor and find a way to shoot it as is. If I can't, I will sell it somehow. But in the meantime I will learn something about old rifles at least - other than that they are pretty expensive. I got some new calipers today and the chamber cast shows the outside base diameter of the cartridge where it joins the rim is .530. The neck is .474 and the cast that extended into the barrel from the chamber is .453. The length of the case is 2 1/4. I don't see any case dimensions like this on the Cartridge Corner website case size charts    (You need to Login or Register to view media files and links)
My Shiloh once fired 44-77 brass shows .514 and .462 at the same case locations. I will try to fire form the 44-77 brass I have in the RB and go from there.