Took out a new upper yesterday, and started having failures to feed- bolt would stop about 1/2" from full closed. started checking things over, all seemed fine when doing it piece by piece. swapped out bcg and it ran fine. Best I could tell was the bolt was dragging across the magazine feed lips causing it to lock up. Hadn’t really seen/heard of this before so wondering if our guess could be correct or if there may be something else to check.
Look at your two BCG’s and see if one has a shrouded firing pin and the other does not. The hammer could be catching on the one that is not shrouded.
Need more info!
Are you saying that changing the bolt carriers corrected the problem?
What kind of NEW upper is it? What kind of lower is it?