Can anyone identify what would cause my Glock-23 to do this to spent brass?

This dent is not on every round, but its very evident on about half that I fired.

Along with this scrape pattern, the mouth is dented in at the scrapes. Its on EVERY round I have fired.
I was using Winchester white box, but also tried a few PMC from a buddy, both did the same thing.
No FTF, FTE or stove pipes. The gun is 10 years old with at least 2000 rounds through it so I know its not a break in issue.
My last few trips to the range, I was with larger groups (all with Glock .40 pistols), so no way to know how long its been doing this.

This dent is not on every round, but its very evident on about half that I fired.

Along with this scrape pattern, the mouth is dented in at the scrapes. Its on EVERY round I have fired.
I was using Winchester white box, but also tried a few PMC from a buddy, both did the same thing.
No FTF, FTE or stove pipes. The gun is 10 years old with at least 2000 rounds through it so I know its not a break in issue.
My last few trips to the range, I was with larger groups (all with Glock .40 pistols), so no way to know how long its been doing this.

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