Pictured below is one of a few cases that have started peening when I fire them.  The cases have been fired multiple times in this rifle but the problem has just appeared.

Caliber: .32-40
Bullet: 175 gr spire pt, 10:1 (breech seated)
Powder: 13.8 gr 4227
Primer: Win lge rifle

Now, for some reason, a dozen or so of them (out of 100 cases) are getting peened dents below the rim.  One of the cases has actually showed some peening at the mouth, but the others are getting dented below the mouth.

I would appreciate some suggestions as to why this might be happening.

Thanks

A good friend used to ask "what was the last thing you did before it stopped working?" It applies to everything. So did you change anything? Marlinguy has a good point about lube and dents. Seems a little too perfectly formed but that would be my guess as well. ratseye

As you increase your powder you can get enough blowback that forces gases around the case mouth.

Some soultions are:

1. Use a lower powder load.
2. Anneal your case mouths so they will seal better in the chamber.
3. Slightly bell the mouths so they seal in the chamber before firing.

With my .32 RKS it would dimple my cases when I shot 13.1 grains of H108 pushing a 236 grain bullet.
I stopped this from happening by reducing to 12.8 grains of powder but then found out later I had better accuracy at 12.4 grains.
I believe your load is too hot.

I checked all my loading manuals for using 4227 in a .32-40.

The majority of powder loads for using 4227 in the .32-40 were for JACKETED BULLETS only.

The only powder load to use 4227 with a LEAD CAST BULLET was maximum 13.0 grains with a 196 grain bullet and 13.5 grains for a 204 grain bullet.

I assume your 10:1 is your alloy mix which is too hard and will allow gases to cut around the bullet.

For breech seating we usually use alloys between 30:1 to 20:1.

I use 20:1 which softens over time from aging.

I found that Peening will happen when You use stainless pins for tumbling when You use the same brass over and over.  the pins will cause tiny dents in the brass and upon Firing The micro dents pop out.  Do you use pins to tumble or was the brass used

A tip I got from Ken Lewis is to wipe the chamber with a patch on a short cleaning rod after each shot. If I don't do this some times I will get a deformed case like you show after firing due to some residue left in the chamber.

JS