Mosin M38 - weird denting in cases.

[Dark Skies]

Anybody know why I'm getting these occasional elongated dents in my cases?

I've had this thing for yonks and mostly used steel milsurp sans issues save the usual case corrosion and cracking from the older stuff.

I shot about 250 rounds at Bisley today. Twenty of my cases ended up with these weird dents.
I'm using the lowest starting charge of H4895 with .311 bullets and PPU brass.

There were no noticeable issues feeding or extracting. But come picking them up I noticed some previously fine brass with this elongated dent.
It's very strange as the remaining 230 are fine.

[meles meles]

Try to avoid stepping on them in your high heels?

[waterford103]

Loads too low combined with throat wear allows the gas to escape past the neck of the case . Insufficent pressure to obturate the case . Usually the cases show signs of carbon staining upon extraction.

[Dark Skies]

Loads too low combined with throat wear allows the gas to escape past the neck of the case . Insufficent pressure to obturate the case . Usually the cases show signs of carbon staining upon extraction.

Ah. So stepping up the load might actually put an end to this, you reckon?

The bore is excellent shape with really strong rifling. Hopefully the throat is in similar shape. I noticed when reloading that the cases seemed a little harder to work than expected. Maybe it's a combo of quite rigid brass plus lowest load.

[Dark Skies]

Shoots pretty well for an old battle rifle. This is a 12 inch Shoot n See target shot from 200 yards with standard front iron sight and a MoJo rear peep sight. At that range the slightest movement causes the big front post to obscure the target. I was pretty chuffed for my first starting load. Think I'll get the front sight to match.

[PeterN]

I had exactly the same thing once with a factory FNM 6.5x55mm cartridge. I still have it somewhere. As above, I was told it was gas escaping past the neck to the rear before the case expanded fully. The escaped gas has nowhere to go so as pressure builds it is trapped in the chamber causing the dent. The case was all sooty also. Maybe a round with a light charge. The others in the box were OK.
Regards
Peter.

[Sim G]

Can’t see how that could be cause with no sooting around the neck and shoulder which I have always seen in the past. And only 20 out of 250? We’re you extracting and ejecting the cases as you should, ie working the bolt with vigour? (Pet peev of mine watching someone draw the bolt back on the back of their thumb to the remove the case with ring and middle finger in some Tom Cruise Hollywood Sniper Shooting. He did this in one of the Reacher films then four blokes down the club started doing it!!)

Anyway, if you are ejecting as you should, I reckon the cases are are swivelling around and bouncing off the back edge of the ejection port on the way out…

If you’re not doing that, I don’t have a clue…

[Dark Skies]

Can’t see how that could be cause with no sooting around the neck and shoulder which I have always seen in the past. And only 20 out of 250? We’re you extracting and ejecting the cases as you should, ie working the bolt with vigour? (Pet peev of mine watching someone draw the bolt back on the back of their thumb to the remove the case with ring and middle finger in some Tom Cruise Hollywood Sniper Shooting. He did this in one of the Reacher films then four blokes down the club started doing it!!)

Anyway, if you are ejecting as you should, I reckon the cases are are swivelling around and bouncing off the back edge of the ejection port on the way out…

If you’re not doing that, I don’t have a clue…

That'd be some vigor to dent these substantial cases! These are dented as if they've been in a machine press. Given the tons of pressure the breech is exposed to I'm going with the low starter load / escaping gas blowing past explanation. Especially as the other cases could be described as slightly sooty.
It may only have happened to a small batch of cases because their tolerances were a tad on the stiffer side, I guess.

[AL8]

I had a similar experience with my K31, it turned out to be not pulling the bolt back with enough vigour.
The case was not clearing the rifle properly

[Dark Skies]

An update. It WAS the lower pressure blowing past the case. I reloaded the undamaged cases plus a hundred more once-fired of the same make (PPU). I upped the charge by 1.1 gr to 44.1 gr (H4895) pushing a 176 gr .311 boatail. Took them down to Bisley today and zero dented cases. Calling that fixed.