Crown
The very end of the barrel's bore, the last surface the bullet touches as it exits, where any irregularity costs you accuracy.
The crown’s job is to release the bullet symmetrically. If the bore exits unevenly (a ding, a burr, asymmetric machining), the propellant gas blows past one side first, pushing the bullet’s base sideways at the most critical moment of release.
Match barrels usually get a recessed 11-degree crown that protects the bore mouth from damage and exits gas evenly. A damaged crown is one of the most common causes of an accurate rifle going sour overnight.