For ~25 years I used a Shooting Chrony, this gave ball park velocities, and generally kept loads safe.

The past few years I've used a LabRadar, accuracy claimed to be 0.1%. This combined with QuickLoad assisted me in getting spot on barrel times, safe working pressures, the ability to tune a load to compensate for differing powder lots, throat erosion, plus other variables that effect pressure and therefore barrel time.

Internal and external ballistics are a math problem, to solve, you must start with an accurate velocity or all down stream equations are skewed.

This has really helped with load development and calculating external ballistic charts.

Generally 10 rounds will find a load that is best the rifle is capable of by the above methods.