If the bullets leave the barrel at the right place on the muzzle axis upswing, slower ones will strike the same place on downrange targets as faster ones.
Tuners change the muzzle axis vibration frequency to make that happen.
I have two chronos. One is an old Pact PC 2 that I use when working up and testing loads for handguns. The other one is a Magneto-Speed V3 which I use for working up and testing loads for rifles.