Buy a copy of: Modern Advancements In Long Range Shooting, Volume I by Bryan Litz.
Then read chapter 15, Measuring Muzzle Velocity.
Spoiler Alert: The top two choices are the Magneto Speed and the Oehler 35P with 4' spacing.

I own and operate both and Bryan is correct.

Anyone involved in the long range shooting business really needs to read everything Bryan Litz has written.