Throat
Also: Leade
The short unrifled or partly rifled section just ahead of the chamber where the bullet first meets the rifling as the round fires.
The throat, also called the leade, is the transition zone immediately ahead of the chamber and just behind the full-diameter bore. It is where the bullet first contacts the lands and grooves, and its tapered shape guides the bullet from the case mouth into the rifling so engraving begins squarely and evenly.
Throat length and angle have a direct effect on accuracy and on how a cartridge must be loaded. A longer throat includes more freebore, the unrifled run the bullet travels before engaging, which changes how far the bullet jumps to the rifling. Because the throat sets that jump, it is one of the chamber features handloaders work around when tuning seating depth.