I was using a filler know as Puf-Lon and a ball powder known as DP-85, an Accurate Powders surplus, a slow burner similar in burnrate to Ramshot's Hunter.  The bullet was an Eagan design, the MX2-30H.  I'd figured on putting the Puf-Lon on top of the powder with Lee measures, but didn't know which one was needed.  So I picked one, oops too much, it filled the case to the mouth.  Tried a smaller dipper, that's better, it filled up into the neck just a tad higher than where the base of the bullet would be, just a little compression to hold things in place.  Not wanting to try to dump the overfilled case cause I'd loose the powder too, I pushed the filler down a bit with a small bolt.  It was a bit more compressed than the others.  That particular round is the one that did it.  It appears it compressed even more behind the bullet as the powder started building pressure, but before the bullet, which was seated into the rifling, started moving.  Right at the base of the bullet, maybe 1/2 of the way down the neck is where the ring is.  I didn't notice it immediately and it isn't deep enough to hinder extraction.  It still doesn't, but I don't like the thought of having weakened steel right above my left hand, so I'll likely rebarrel it before passing it on to my grandson.  The load wasn't over pressure otherwise, it just compressed that Puf-lon over the barrels yield point.  I can't remember the exact charge of DP85, maybe around 35gr.

I'd add with Krag barrels.  The lack of chrome moly isn't a help to them, they are softer than barrels made of modern steels..  I have however, even read of some Garands having chamber ringing problems with ball powders, not with filler.

I just dug out Sharpe's 1937 book.  He gives pressures for 30-06, 110 grain bullets and non-corrosive powders for both Unique & 2400.  Max load for Unique is 19.8 grains, 40,000 psi.  Max load for 2400 is 29.0 grains, also 40,000 psi.

Once in a great while, I shoot a .401 WSL, a 1910 blowback.  The recommended load with 200 grain bullets has always been 24.7 grains of 2400, @ 50,000 psi.  That load blows the breechblock back so fast that the extracted & ejected case gets pinched in the ejection port by the returning breechblock.  I switched to 4227.  Still like swinging open the Gates of Hades, but it ejects properly.

My guess is your neck ring developed because the 2400 load developed peak pressure before the bullet started to move.  Plus the steel in that 1917 barrel may not be much different than the steel in Krag barrels.

In today's world, I cannot imagine Sharpe's Complete Guide to Handloading in a public library.  But it is not too hard to find in a decent used book store.  There are 2 editions, 1937 and 1941. The 1941 edition has a chapter by Sharpe on military salvage and a chapter by Ned Roberts on Schuetzen cartridges.  I think it is worth the chase.

For those more computer literate, the copyright ran out years ago.  The 1937 edition is available on the internet, IIRC as a pdf and free.  I think it is also available as an e-book for a minimal charge.

The 1937 edition of Sharpe's book is available online at (You need to Login to view media files and links)

Still interesting reading.