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Just throwing this out there but, it might be that you have a weak magazine spring that is not throwing the shell far enough back onto the carrier that is jamming up the gun. You will notice that when the bolt is locked back and you press the bolt release it causes the carrier to flip up into the loading position. Possible with your heavier loads, the recoil is helping to nudge the rounds further back onto the carrier and that's why you don't experience the failure with them. With the lighter loads, does it happen on the last couple of rounds from the mag when the spring is less compressed? It's worth looking into.