Fouling
Residue left in the bore after firing, carbon from burning powder and copper from the bullet jacket. Excess fouling can open groups and shift velocity.
Carbon fouling is normal and predictable; copper fouling is more variable and can accumulate quickly in some barrels. Symptoms of excess fouling: rising pressure signs, dropping velocity, opening groups.
Cleaning intervals vary by rifle and load. Some precision shooters clean every 30-50 rounds; others let it ride to 200-400 rounds. The honest test is the data book. Track groups across rounds since cleaning and stop when accuracy drops.