Carbon Ring
A hard carbon deposit that forms at the case-mouth and throat junction, where it can raise chamber pressure and quietly spoil accuracy.
A carbon ring is a band of hard, baked-on carbon that accumulates right where the case mouth meets the throat, at the front edge of the chamber. It forms because combustion byproducts collect at that step and bake into a dense layer over many rounds, and unlike soft powder fouling it resists ordinary brushing. Left alone it narrows the bore at a critical spot.
The danger is that a thick ring effectively reduces clearance for the loaded round, which can spike chamber pressure and degrade accuracy without any obvious warning at the muzzle. Because it sits hidden inside the chamber, the reliable way to catch it is with a borescope, and removing it generally takes a dedicated carbon solvent and patient scrubbing rather than a quick pass with a bore brush.