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Turns out a piece of debris had worked it's way on top of the primer ram on my Dillon Square Deal B and was causing these little dents. Obviously it wasn't enough to make one pop, but boy did I feel stupid when I saw I'd churned out about twenty of them and I wasn't wearing safety glasses. ALWAYS wear safety glasses when reloading.
I fixed it by with a quick dry scrub with a toothbrush (the wife's, not mine).