An old German gunsmith taught me an old world way of dealing with errsion on barrel set backs. If you can find a point where the bore dia matches at the muzzle, that does not wear, with the position of the new throat, you can have great success with the barrel set back.
Again, the muzzle does not wear past the first 1/4-3/8". Using A grizzley rod and reamer pilots that are graduated in .0002 increments, determine the dia at the muzzle. Next, using over size reamer pilots on the Grizzley rod begin measuring wear on the breach end. Put a piece of masking tape on the barrel and mark the distance that each over size reamer pilots penetrates the bore and the location. Find the exact point in the barrel where the "new" throat location will match the muzzle dia. Get ready for a shock at how much has to be cut off.
Set backs with max heavy varmint contour barrels along with heavy Palma can be accomplished resulting in great accuracy results.
It is not uncommon to cut off 9".
I just cut off 9" from a 6BRA of mine, MHV contour. The barrel started showing a shot at 1100 rounds of 105s shot with H4895. Also, I bought a used 6.5/284 fluted barrel with 600 documented rounds. I got the barrel cheap and thought I would take a chance. I determined that the best I could do was to locate the new throat at a place where it was .001 over the muzzle bore dia., bummer....it is going to be a short range hunting barrel in 260 AI. Also to the barrel's detriment, the Muzzle dia and the breach dia are very close but under the fluting, the bore dia opens up to a dimension that I can not measure accurately. The whole project will end up being a lot of work for a fire form barrel, but I got to play. I will buy a lottery ticket if the barrel will shoot 1/2".
Cutting off a barrel, helter skelter without taking to account the bore dia, is a Hail Mary at best depending on your desired accuracy.