Reaming a finished barrel.
Don't even think of it unless you want to ruin the rifling in your barrel.
Let me get this straight. Your bore diameter runs .457 but the rifling lands run .446 instead of .450, a difference of .004 inch. Because of the tighter bore diameter, you are having difficulty in seating bullets.
First of all, a reamer is NOT the way to go. Reaming the bore would result in putting a rough finish on the present rifling, which you then have to lead lap. If you do not completely smooth the bore out by rifling, then small burrs are left on the rifling in the direction of the rotation of the reamer. Not good!
What we are looking at here is a difference of .004, or a difference of two thousands of an inch on the sides of the bullet. If you are shooting Black Powder, you are probably using lead alloy cast bullets, so that difference of four thousands of an inch is not really a factor. In fact, it is a bit better, because the rifling gets a better grip on the bullet.
As an example, the original German 8 mm Mauser (8x57 I - in German 8x57 J) was a .318 bore diameter with a round nosed bullet. When the Germans changed to a lighter spitzer bullet, they changed the bore diameter to .323 and designated the cartridge the 8x57IS (or 8x57JS---the S being for Spitzer bullet and .323 bore), they left the original rifling lands the same diameter as the original cartridge, but simply made the bore bigger. This is why German Mauser Military rifles have such a prominent rifling. The difference was .005, but they still shot metal jacketed bullets out of them, at higher velocities, even though the rifling was deeper.
You do not need to ream the entire bore. Just find someone with a proper reamer for the 45-70 or a 45-70 throating reamer, and open up the throat (or leade as it is sometimes called) so that it will take your loaded cartridge.
A very simple and inexpensive solution to your problem.
I would not open up the throat TOO LONG, because if there is too much jump before the bullet reaches the rifling, accuracy could suffer. Remember that such shooters as Pope used to have a seater that would load the bullet first INTO THE RIFLING and then they would put a loaded cartridge case only into the chamber. Two seperate operations. They shot from the standing position, and look at the targets and groups they made!
In fact, many of the Custom Riflemakers will ask you what bullet you intend to use, so that they can throat the barrel for optimum performance with that particular bullet. When you go to see your gunsmith, take along a DUMMY round with the loaded length you want, so he can fit the throat of your rifle to that round.
So, if you want to bugger up your barrel, ream it full length. If you want to shoot it with some accuracy, then throat it to take your particular bullet.
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