Barrel Harmonics
Also: Barrel Whip
The vibration that travels through a barrel when a shot fires, flexing the muzzle in waves that the bullet must exit at a repeatable point.
When a cartridge fires, the barrel does not stay still: it flexes and whips in measurable waves as pressure and the bullet travel down the bore. Barrel harmonics describe those vibrations, and they matter because the muzzle is moving while the bullet exits, so any shot-to-shot change in timing throws the point of impact around.
The goal is to have the bullet leave the muzzle when the barrel is at a steady, repeatable point in its cycle, often called a node. Shooters reach that node through load development, adjusting charge weight and seating until the groups tighten, or by adding an adjustable muzzle tuner. Barrel stiffness from a heavier contour also calms the whip, which is one reason heavy barrels are easier to accurize.