Throat Erosion
The gradual wearing-forward of the throat under repeated heat and pressure, which moves the rifling away and slowly degrades a barrel's accuracy.
Throat erosion is the wear that builds up just ahead of the chamber as a barrel is fired. The intense heat and pressure of each shot, plus the scrubbing of powder gas, slowly roughen and consume the throat, pushing the start of the rifling forward and opening up the bore in that critical first inch.
As the throat erodes, the bullet has to travel more freebore before it engages the lands, the jump grows, and groups eventually open beyond what load tuning can fix. That progression is what defines practical barrel life, and it accelerates with hot, overbore cartridges and high round counts. Erosion is distinct from fouling, which cleaning removes, because eroded steel is gone for good.