I have two uppers, one CMMG and one RRA. Both have run flawlessly with steel case ammo or the brass. I did have a problem with a batch of wolf steel cased ammo a while back where the cap strike was not firing the round, but tried it through the AK with the same results, so sent all 1000 rounds back to wolf, (come to think of it still have not gotten that resloved yet). I also have the 5.45 upper, run them all on the same lower with no problems. These are all 16" or better barrel lengths, do not know if SBR would change the outcome.
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