+1 on the copper residue...see how it follows the edge of the land. Maybe yours was one chosen to test fire or something like that. VMaxsplat was right I would break the barrel in (I personnaly dont believe it has to be rapid fire shots) 100 rounds should do it. Use the time to get your sights really adjusted and get the feel for your rifle but I believe it is copperfouling. I used borebutter when breaking in my barrel but there are a lot of things you can use. I would clean it and then run a patch with a little bore butter on it through it and then go shoot it. I helps to season the material and I thought that if it worked real well on my Hawkins it would work on my AR...

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Grover Oathkeeper