The Transition Zone
The throat, also called the leade, is the short section of bore between the end of the chamber and the start of full rifling. It guides the projectile out of the cartridge case and into the rifled bore. Because it sits at the boundary of the chamber standard, it is a defined dimension in technical drawings rather than a typical headline specification.
Throat and Leade Defined
The throat is the length of bore ahead of the chamber mouth in which the rifling is either absent or only partially formed. The leade is the tapered ramp within that region where the lands begin at a reduced height and rise to full profile. Together they form a lead-in that eases the projectile from case to bore.
Freebore
Freebore is the portion of the throat that is bored to roughly groove diameter with no rifling at all, so the projectile travels a short distance before touching any land. A longer freebore lets the projectile move forward before it engages the rifling. Freebore length is one of the dimensions a chamber drawing specifies.
Leade Angle
The leade angle is the taper at which the lands transition from their reduced starting height up to full engagement. A gentler angle spreads the engraving over a longer distance, while a steeper angle engages the projectile more abruptly. This angle is part of the standardized chamber geometry captured in the drawing.
Jump to the Rifling
Jump is the distance the projectile travels from its seated position to first contact with the rifling, set by the combination of freebore and how deeply the projectile is seated in the case. It links the fixed chamber geometry to the loaded cartridge. Because jump depends on both the chamber and the cartridge, the same chamber can present different jump values with different loads.
Worked Example
Consider a chamber with 0.050 inch of freebore. If a cartridge is loaded so the projectile's front bearing surface sits 0.030 inch behind the case mouth reference, the projectile must travel that gap plus the freebore before its bearing surface meets the lands. Seating the projectile farther out reduces that jump, while seating it deeper increases it, all within the one fixed throat.
A Common Misconception
The throat is frequently confused with the chamber itself or with headspace, but it is a distinct zone lying just ahead of the chamber. Headspace concerns the surface that stops the case; the throat concerns the projectile's path into the rifling. Because it appears in dimensional drawings rather than consumer specification sheets, many readers never encounter the term even though it is fully standardized.