It would be a little hard for me to dig mine out of the safe now, but I think mine has a similar mark. Even if its not common, it nothing to worry about, the metal hasn't lost any integrity imo.
Six is right. That's not an area where Colt marks AR-15s.
Unless it's a burr that is above the surface I really wouldn't worry about it. It should not affect function under these circumstances.
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