This question came up on another Forum. I didn't care to get involved on that forum, but, the OP had an interesting dilemma.

Described Situation:

Recently purchased, nice looking, original finish, U. S. model 1898 Krag rifle.
(The owner is a general collector of U. S. arms and wanted one Krag in his collection, to shoot).
The bore cleaned up Very Good, except, the last 1/4 inch at the muzzle. That part of the bore is badly pitted to the point rifling is non-existent.
During initial Range Session, at 50 yards, this Krag spread shots over a 6 inch group.

The OP is considering having the Krag barrel counter-bored 3/8" to get to good rifling and create a sharp 'crown'.

If this would destroy the value of his Krag, he would rather just sell it, as is, and look for a better shooting Krag.

(Assuming the OP used good ammo, is a competent shooter, and the Krag is original and collectable):

What are the thoughts of KCA members?

FWIW - (If it were mine and the bore looked really promising, I would likely go the counter-bore route).

I have been watching this thread as well. Sell it to a person that wants a wall hanger and buy a more shootable rifle. To counter bore would reduce collector value and resale value. Krags with quality shootable bores are still out there, no sense messing with a nice collectable rifle

I have no input on collectors value, but what exactly is counterboring as applied to a Krag?
I have seen a report of a test on crown angles and changing to all the trick angles made no difference in accuracy on a rail gun. So doing a large counterbore like .375 diameter and .375 deep to make an 11 degree crown inside the 3/8 hole would be a waste of time and really show something had been altered.
Seems like one could bore or ream the last 1/4 inch to plus .002 over the .308 bore taking out all of what is left of the rifling and leave the existing crown. Looks would not hardly be changed except the last bit of the bore would be shiny with no rifling. Maybe the pits are deeper than that?

I imagine opening the last 3/8 inch of the bore to .310" to .315" would remove all contact with projectile. A clean square shoulder, where the rifling ends, would be the new crown.

IMHO - The removal of the damaged area of rifling, at the muzzle, would hardly be noticeable. A skilled machinist could do this.

Some of the M1 carbines, that came back from Korea, had this done to remove muzzle-wear and improve accuracy. It seems to work.

I would make the internal transition at least 45 degree angle as a square corner will break down quicker and catch cleaning patches. Like you said, it would be modified but hardly noticeable.
 
Set up and zeroed in on a lathe a 5/16 reamer would give you .3215 with a 45 degree lead and an 8mm reamer would give you .315 with the same lead angle. Done instead with a single point boring tool would give 100% concentricity to the existing rifling. Reamers might want to follow the existing erosion/wear unless cleaned up with a boring bar first. Your gunsmith will know all this already.

The Russian Arsenals countered bored Mosin Nagant 91/30 Rifles probably by the millions.  I have 5 that are counter bored. They are bored to about a depth of 1 to 1.25". The new internal crown is quite square to my eye.  If you look at the end of the barrel it is hard to tell unless you look real close. The ones I have were bored out to around .325 to 327. The counter bored rifles that I have shot have more than acceptable accuracy for a 100 plus year old rifle.

If the OP's Krag is not real collectable. a counter bore will likely make it a much better shooter.

Zgun

I've got a barrel set aside to experiment on.  It had a mud dauber's nest in the last 1/2 inch but an otherwise nice bore.  Figured I'd put it in a receiver and do some before and after counterboring shooting with it.

Your 'experiment' will be of interest.