What causes dents in cases below the shoulder like these

[Bullet bumper]

Could be from excess oil in the chamber and the pressure of firing causes some detonation ( rapid oxidization of the oil under pressure ) .
This is why bullet swagers use lubes that don't detonate .

 

[RockyMtnMT]

I went back and read the original post again. Hard to say where it happened if it was not noticed until after tumbling. He just needs to load them and shoot them again to get them back into shape. If it is still there after being shot then we could say it is happening in the shot. Then look at annealing.

Steve

 

[LaHunter]

Too much lube.
This will absolutely happen from too much lube, even when using a lube pad.
The OP stated he noticed after FL sizing.
A little lube goes a looooong way.
I try to keep the case shoulder off the lube pad, which helps prevent these dents.

Also, after about 50 re sizings, it is a good idea to clean the sizing die with a residue free degreaser/cleaner.
You can get case lube and neck lube accumulating in the die, and these dents will happen. I know this from experience.

Good Luck

 

[Hawk in WY]

Those are exactly the dents I see along with case head separation.

Check the inside of those cases with a bent, sharpened paper clip.

I would not fire those cases again without checking for case head cracks.

Safety first, always.

 

[atl5029]

Thanks for all the info guys. I cleaned my die and haven't had any more dents.

 

[Bullet bumper]

The Op stated he does not know when it happened . Could be after firing could be after sizing .

 

[Kansaswoodguy]

1 part 100% liquid lanolin to 10 parts 99% isopropyl alcohol couple of squirts from a cheap spray bottle lubes outside of cases and inside the neck. Best and cheapest case lube I've ever used.

 

[GTN]

I have a particular FL sizer that does the same exact thing. It is definitely to much lube with mine. I use a the least amount of lube with it I can possibly get on the case and all is well. I clean the die regularly as well.