I'd say time for reduced loads until one finds a new bolt.

What is the load?

Any chance those are pistol primers?

How rough is the bore?

Can you feel any unusual spots as you push through a tight patch?

The too etched spots show a history of leaking, were they there when you got the gun?

bolt face insert. Note the annular ring. That certainly intrigues as I don't remember hearing that the Arisaki bolts were fabricated in that way.

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Sincerely,

Hobie

The one Remington primer that pin holed on me years ago was from a starting load of an 85 grain Sierra JHP over 33.0 grains of 3031. Not a hot load at all. The bore on my Arisaka looks like 22 miles of jeep trail, but it still shoots well as long as I don't try to push the velocities too high. Those little 85 grainers seem to like each other and scrooch up real close together when I shoot them at moderate velocity, but things get squirrely when I try to push them much faster. The Norma 139 grain SPs shoot nicely too, but I have tried several other bullets that don't do as well. I tried the Hornady 129 grain spire point some years ago and never could get it to shoot well.

As I mentioned, Remington recalled several lots of those primers because of metallurgical flaws in the cups that caused pinhole blowouts. That was several years ago.I have not heard of similar flaws in other primers, but that is certainly a possibility here.

Did you check headspace yet? I went to a gun show and looked at some Ariska's there. None of them exhibited pits like yours, though most did show an annualar staining similar to yours. My best guess is for some reason your loads are developing excessive pressure and the pits are allowing the primers to let go in those locations. I would seriously consider having a very competent 'smith inspect it and possibly take a chamber casting to see if there is a dimensional anamoly.

As Catoosa knows I have several 38s or variants and every one of them has an eccentric chamber bad enough that resizing would "wear out" the brass pretty quickly.
That led to what CAS alluded to.

I had a 14" 6.5 TCU barrel and did not want to pay for the dies to make into the 6.5 jdj.

I got to looking at case capacities and read an article where a gunzine writer thought the 6.5X50 was good in a Contender, so off to JDJ for a rechamber since they listed that as a reamer on hand and I had that brass and dies sitting around slowly returning to their natural state .

I got it back, shot it a little and got flattened primers and 6.5 TCU velocities with 10 grains more powder(as I recall way back data) than the original 6.5 TCU.

That is when I think it tied into the story CAS talked about, Rich decided 6.5X50 would be a great chambering and posted his idea in a group we both belonged to.
I offered him brass, dies and barrel to test and see what he found, but he had to do a write-up of his trials and tribulations (still out there on the net I believe). He went to greater efforts than I did and found the same basic disappointments, but at least he was only out time, powder and bullets. AND, hey shootin is always a reward in itself.

He sent the barrel and supplies back and I received several suggestions of a change to get rid of the tapered chambering which most believed to be the issue, 6.5X54 MS would clean up the chambering and is pretty straight sided.

It has languished in a dark place ever since and only recently have I considered the MS or 6.5 Super Bower to clean it up and make it useful. Not even sure if the Super Bower is available now.

Neither of us tested 100 grain bullets and so since I have a bunch of 100 grain BTs, may try them and them before I make a decision what to do.

Like Catoosa, the barrels may not be pristine and yet they all shoot pretty decently, in fact I got one deer last year with my TRUSTY 6.5X257 that John Hannah and I built in about 67-68 (1 of 2 which one of my ex's relatives absconded with the other while I was out of the country for a while).

I did handle a 6.5X50 Carbine with walnut . . . yes it was two piece, but walnut stock that was pristine inside and out at a Nashville gun show.

Wanted 150 for it and to this day I regret buying it. Regret not buying the imports from China where they had rebored and rechambered them to 7.62 x39, but I amguessing there are many nonpurchases from the past that we regret today.

Just to keep this sort of "on Topic" I really prefer guns with that roundy thing that carries the cartridges around in a circle to unload . . .
With a resounding bang of course!