How it works
Fire one confirmed shot at a known distance and tell Atlas where it actually landed relative to your call. From that single data point, Atlas reverse-solves either your true muzzle velocity (the speed the bullet is actually leaving your specific barrel at, which almost never matches the box) or your true ballistic coefficient, which accounts for how your specific bullet is actually flying compared to the published drag model.
Once trued, every future solve from that rifle and ammo profile uses the corrected number instead of the factory estimate.
Why it matters
A box muzzle velocity that's off by 30 to 50 fps is common, and that error compounds with every hundred yards you stretch the shot. Truing removes it with a single real shot instead of a guess.