Built a new rifle with an 18" rifle length barrel. Cleaned the barrel initially, noticed my patch holder was getting slightly shaved. I looked inside the bore and noticed the gas port looked just a little odd to me. No noticeable burr. Shot it, breaking in, cleaned and done for the day, no burrs, port looked the same, bore was a mirror. Now I have about 200 rounds through (all black hills 77gr smk) and I have what looks to be a smudge starting at the port out towards the muzzle, and then it ends about 3/4-1" before the end. It follows the twist of the rifling, but looks gritty... I have never seen anything like it. I zeroed and have just shot paper at 25 yards, so I'm not sure if it is affecting accuracy at all yet.
Cleaned with a brush. It did turn out to be just stubborn build up. Shot a few times after, still close range, patched it out, no need for a brush. I will shoot longer distances in a little while and report back for accuracy.