Indentations on 9mm case - Ammunition & Reloading - New Jersey Gun

Ive started cleaning brass and have come across a number of cases with these indentations on them. They go all the way around, the depths of the indentations vary as they go around. They also vary from case to case. So far about 4-5 out of the dozen or so Ive pulled out of the tumbler have them.

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