What caused my case to do this? (Help)

[devildogandboy]

I would think and this mho, if there was gas leakage wouldn't the indents be darkend a little like it is around the case mouth? And why would the shoulder be unharmed and the dents appear after (behind) the shoulder?

looks as if he may have wiped the cases down. i don't know.
i agree there should be powder residue in the dents. as for the shoulder area i can't say.
believe me, i'm no expert, just giving my ideas lightbulb and opinions. i learn something new everyday.

 

[Loner]

I would say that is a bit of condensation collecting in the bottom of the chamber and
getting pushed forward on the round being chambered. Case looks way to clean for
powder to be causing it.

 

[tinman13kup]

I don't know. I'm having a hard time believing the pressure that escaped past the neck was enough to overcome internal case pressure.
Were the rounds full length sized? Have you tried running them through a sizing die to see if the necks are out a bit? Have you checked case length? It does look like a hydraulic dent from sizing lube. I've run into some issues with lube carbonizing in the chamber and cementing rounds in, but that's after a few rounds in an AR-10. It also leaves carbon goo on the brass when you pull it out.

 

[meatyrem]

I would say that is a bit of condensation collecting in the bottom of the chamber and
getting pushed forward on the round being chambered. Case looks way to clean for
powder to be causing it.

I'll have to go with Loner on this