Freebore

[3][4] The chamber is the rearmost portion of a firearm barrel that has been formed to accept a specific handgun/rifle cartridge or shotgun shell.

[5] For firearms having a rifled barrel, the bullet typically extends forward from the leading edge of the cartridge case.

The leade is the tapered section of the throat that transitions in diameter from the freebore to the rifling lands at a shallow angle, typically between 1° and 3° degrees.

[7] Freebore length affects the distance the bullet will jump before engaging the rifling when the weapon is fired.

[9] Dimensions of freebore length and diameter may gradually increase as hot gas wears the interior barrel surface each time the weapon is fired.

Chamber illustration indicating the various sections of a typical rifle chamber. The freebore is the cyan colored section just ahead of the neck.
Closeup of chamber throat depicting relationship between freebore diameter, rifling groove diameter, and land diameter.