This can happen if the gun was fired much with the gas cylinder plug loose. Carbon builds up in the valleys of the threads on both parts, but is biased outward by the gas direction. Unscrewing the thread is easy, but screwing it in loosens bits of the hard carbon residue and it then jams on re-entry. I don't know why this isn't equal and opposite on the plug and cylinder, but may have to do with slight differences in the cast and billet steel causing the carbon to stick to one better than to the other?
I recently started in on two CMP Garands. In both cases some considerable carbon was present in the gas cylinders and on the plugs. I tried the Slip 2000 gas cylinder cleaner for the first time and found that it performs as advertised; actually softening the carbon into sludge that can be brushed out. You might try this product if any carbon traces persist? They can be hard to see down in the thread valleys. I don't know of anything else, including carburator cleaners (I've tried many, inlcuding the strong immersion soaking Berryman's), that get carbon out the way the Slip 2000 product does. Nonetheless, to find out if this is the problem before ordering the cleaner, try spraying the gas cylinder threads and the gas plug threads with a strong penetrant, like PB Blaster, then running the plug in and out a few times, using the ratchet if necessary. Then wipe it all off with a white paper towel. If carbon in the thread valleys is the villain, you will see at least some black come off on the towel.
Also, check to see whether there is a gas leak channel apparent in the threads. If so, the parts need to be replaced if you don't want the problem to reappear. If you don't mind the reappearance, then don't worry about it. This presents no hazard; just a cleaning nuisance.
Nick
