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stay away from cheap steel cases unless you like swapping extractors.
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We've run almost nothing but steel our AR's and have had no such problems at all through many, many thousands of rounds. The bigger issue with surplus rounds in a 7.62x39 AR is hard primers with a standard AR firing pin which is slightly too short to reliably ignite them. A heavier hammer spring has worked good for us, though I know some guys actually modify their bolts by taking a few thousandths off the end too allow a little deeper penetration of the firing pin for those harder primers.
The biggest issue you'll have is finding reliable magazines, because they're just a lot more scarce on the 7.62x39 AR platform. There's a great sticky posted
here on this caliber in the AR that will probably answer most of your questions and help give you some other ideas as well.