Cartridges are bending?

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In all fairness the bolt may have been catching in the case extractor groove to push the round forward but I don't believe it was fully behind the case when the feed failure occurred.

I too, have had that, when using a h2 buffer and extra power spring in a lightly gassed and very dirty rifle. Mangled the round when a stoppage occurred. Did it with every Gen PMag, both 20 and 30, along with USGI 20’s and 30’s and Surefeeds. Did it mainly with .223 55gr, and didn’t start until I started shooting .223 (I usually shoot 5.56.). It was persistent until I swapped in a lighter spring. It certainly appeared to be what you are describing.