The proof mark on Krag barrels supposedly showed that after production testing was done successfully with a load rated at 70,000 psi. PSI and CUP ratings are somewhere around 7% of each other, + or -.

So, the "safe" rating of 40,000 CUP does not mean that things start blowing a part at 40,001 CUP. There's some room to spare, Mr. Petrov's experiments not to be ignored, though.

I have been hanging around here long enough to know the Krag is relatively strong and long lived if common sense is used.
I got the impression the seller had been feeding hundreds of rounds of 303 through it. Finally looked at the ad and 3 rounds. He bought it from someone that thought is was converted. Maybe he is just an idiot for goofy advertising, instead.

I have never touched a drill to a Krag receiver, but it has been described by a gunsmith that I trust as hard on the case and butter soft under the case. Looked at the fellows gauging of the barrel with a 303 round, I would guess that barrel is closer to. 308 than . 311 or .313. All that said if 303 rounds are loaded to 49K Cup then this thing has operating at 30.06 pressure levels for some time. Butter soft steel will move under abuse even though no cracks show. I think this would happen mainly in the receiver as the bolt treated with cyanide on the lug would be harder since more surface area on the angles and corners would have been benefited from the extra carbon.

My worry was that increased head space could lead to a separation in the extractor area. Looked at a loose Krag barrel and that is pretty unlikely. If the headspace is stretched on this one, a person could keep on using 303 brass with 308 bullets and stop further damage. It is a shame this was done to an otherwise nice looking rifle. Maybe the first owner that did the 303 thing, only shot a few rounds and damage is minimal to non-existent. I would pass on it because of the unknown.

This is the same kind of tabulation that shows there is a + or - 7% variation between these two pressure unit determinations.

Good discussion about concerns for over-pressurizations and re-loading concerns with these older firearms.

As always... Para comes through with the good stuff.  Many thanks Para!

This all boils down to misinformation, misunderstanding, and downright confusion.  As FredC says, there are too many unknowns.  Was it REALLY chambered for .303, or did some unsuspecting or uneducated shooter simply not know the difference (to the untrained eye, at first glance, one could easily mistake .303 for 30-40).  Without actually speaking to the first guy sticking a .303 into it... and without actually measuring the chamber/barrel/receiver/etc., there are too many things left to chance!

I've been given several varieties of .303 British ammo but I have no .303 British rifles.  So most of the .303 got sized down in my tightest chambered Krag and it's now emergency Krag ammo, or it could still be used in .303 since headspace is on the rim.  I'd use it in a sporter that has a .3105 groove diameter barrel if I needed to.  Most is hunting ammo, but there's also some South African ball of WWII production.