Forcing Cone
The tapered transition between the chamber and the rifling, also called the throat, where the bullet first engages the lands as it enters the bore.
The forcing cone (sometimes called the leade or throat) determines how the bullet enters the bore. A sharp, well-cut forcing cone aligns the bullet centrally with the rifling so engraving happens evenly on all lands.
Throat erosion (the slow widening of this section under repeated firing) is the most common reason a precision barrel goes off. Match barrels usually last 1,500-3,000 rounds before throat wear makes them inaccurate at distance.