I would say that either the wrong sizing die was used. The carbide ring is missing. Or the sizing die was not set up properly. Maybe steel cases? Cases fired in a Glock has nothing to do with it. Me and 3 other people that I shoot with. We all use mixed range pickup brass, and we all shoot Glocks. With well over a million rounds fired of range pickup brass. You might have problems with range pickup brass that have been shot in some full auto machine firearms. But just because it was fired in a Glock has nothing to do with anything. Unless you are pushing the upper limits of max loads. And that could be with any pistol.
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