Sport45, the biggest variable in the way most folks use chronographs is themselves. Us humans don't hold a rifle to our shoulder with exactly the same force from shot to shot. Nor is our body positioned behind the rifle exactly the same for each shot.
Clamping a rifle in a free recoiling return-to-battery machine rest is best for rifles. Barreled actions clamped in a solid mount is what SAAMI uses. Arsenals use a barreled action in a Mann rest; a long V block holding the barrel's fitment rings. Such devices have the same resistance to recoil during barrel time for every shot. Velocities are very accurately measured.
US Navy Distinguished Marksman Badge 153
Former USA Palma Team Member
NRA High Power Long Range High Master
NRA Smallbore Prone Master