Could be from someone’s chamber but also I’ve sorted through some xm193bk bulk packs and found tons of lake city rounds/brass that I wouldn’t shoot. Deformations, cracks, etc. wouldn’t surprise me if that came from the factory like that.
You should be able to look in the chambers and see. I would do that ASAP. I would never shoot or depend on a rifle with pitting in the chamber. The brass can expand and stick in those pits and have failure to eject and then your fucked. Never heard of a breach block. That sounds funny.