Bullet Jump
Also: Jump to Lands
The gap a bullet travels from its seated position before it engages the rifling, a distance tuned through seating depth to sharpen accuracy.
Bullet jump is the free run the projectile makes after the powder lights and before its bearing surface bites into the lands and grooves. That gap exists because the bullet starts seated short of the rifling, and its length is the sum of the chamber’s freebore and how the bullet is seated. A short jump puts the bullet close to the lands, while a long jump gives it a running start into the bore.
How a rifle responds to jump varies by bullet design, since some tolerant bullets shoot well across a wide range while others want a precise distance. Handloaders tune jump by adjusting seating depth, often working in small increments to find the window where the rifle groups tightest. As a barrel’s throat erodes forward with use, the jump grows, so loads are periodically re-checked to keep the bullet positioned where it shoots best.