I have been doing some load development in my Glock 20SF Gen 3 and have been getting some brass back with strange indentations on the case mouth. It has not caused any failures but perplexes me none the least. It seems to occur every five rounds or so. These indentions have happened with a 4.6" KKM barrel and a 6.0" Lone Wolf barrel. The gun has a Lone Wolf SS guide rod with a 22# spring  the gun is stock otherwise. The loads I am loading are high power Double Tap / Underwood power loads using 180-230 gr. bullets.

The indentations on the case mouth line up with edges on the chamber hood of the barrel. One side of the case mouth has a slightly wider indentation that the other. See photos.

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