NC 20" Upper AR-.22

[ERNO]

What does "NC" stand for? Does it use a ceiner type bolt?
I have approx. 35,000 rds thru my "Model 1" dedicated 22 upper.The ceiner bolt does not like notched hammers. Approx. every 3,000 rds. the Rock River match hammer would break, including one JP match hammer which broke in half. I use a mill-spec hammer now. I've broken 2 firing pins so far. Spikes Tactical uses beefer firing pins. Around the 35,000 rd. mark it started to double on me. I finally cleaned the extractor by taking out the extractor pin halfway out from bottom up.I believe the extractor stuck in the closed position there-by hitting the rim and causing it to fire. No problems with doubling now. I now try to keep the bolt clean and I now use a 22 bore brush, and a 17 caliber rod for cotton patch work, but I believe I still have lead stuck in the barrel. I might have to use Brownnells lead out screen to get that lead out.