Jam
Also: Hard Jam
A seating depth that pushes the bullet into the rifling rather than leaving a gap, the opposite of jump, used to tune accuracy in some rifles.
Jam refers to a seating depth at which the bullet is set out far enough to contact or engage the lands and grooves when the round chambers. It is the opposite of bullet jump, where a gap separates the bullet from the rifling. A hard jam seats the bullet so it presses firmly into the lands, while lighter jam settings have the bullet just touching or lightly engaging.
Some rifles shoot their tightest groups with the bullet jammed into the rifling because doing so aligns the bullet with the bore and removes the free travel before engraving. Jamming raises chamber pressure and can pull a bullet from the case if a loaded round is extracted, so it is used deliberately during load development rather than as a default. The exact jam length is measured relative to where the bullet first touches the throat, and it shifts as the throat erodes over the barrel’s life.