While it fired 59/60 rounds after the new gas rings and gas tube, that 1/60 bothered me. Well, when I broke it down to clean it, I found the roll pin that holds the rubber bumper in had drifted, protruded about 3/32"+ out of place. That alone could cause a problem with feeding.

That would explain why it functioned 100% before I switched the barrel to the M16A1 upper from the slick side SP1 upper. It had nothing to do with the switch, but rather the displaced roll pin. with the worn rings it significantly affected functioning. Replacing rings and gas tube overcame some of it, but not all. The difference in sound of the round may have been the bolt not opening all the way, rather than light load.

I would bet it will function 100% now.