The idea a tight throat and maybe a rough or coated chamber doesn't sound plausible given he has tried two "identical" 18" BCM barrels with the same results. Plus as Montana pointed out, the test where you drop a live round into the chamber and reverse to see if it falls out freely would have it sticking.
I'm also thinking any interference fit between the bolt and carrier would be noticeable when they swapped and assembled both bolts into the same carrier. It still could be leaking gas at the BCG however if the tolerances were out, or the gas key loose. Not enough to cause a sticky spent case however.
Eye,
I don't imagine the tight thermal fit has anything to do with the problem. If you fitted the barrel all the way to where the barrel extension butts up against the receiver nose, and then cranked the barrel nut to its spec ft/lbs (which pulls the barrel extension in to where it is supposed to be) then all is well. You would have worked the action a few times when assembled and then fired the trigger. If the BCG goes into battery and the trigger can be fired then there's no reason the barrel/upper assembly will be the cause of a fired case jammed in the chamber.
I'm also thinking any interference fit between the bolt and carrier would be noticeable when they swapped and assembled both bolts into the same carrier. It still could be leaking gas at the BCG however if the tolerances were out, or the gas key loose. Not enough to cause a sticky spent case however.
Eye,
I don't imagine the tight thermal fit has anything to do with the problem. If you fitted the barrel all the way to where the barrel extension butts up against the receiver nose, and then cranked the barrel nut to its spec ft/lbs (which pulls the barrel extension in to where it is supposed to be) then all is well. You would have worked the action a few times when assembled and then fired the trigger. If the BCG goes into battery and the trigger can be fired then there's no reason the barrel/upper assembly will be the cause of a fired case jammed in the chamber.