Drag consistency has been discussed more in recent months than I can recall in many years.
The average consumer now has the ability to actually measure it without "breaking the bank".
Two Garmins will give you a MV and a downrange velocity. Hard part is shooting within the appropriate place near but not on the downrange Garmin. I know it has been done at 800 meters. They only lost 1 Garmin.
I've heard about the Shot Marker system but don't have any experience with it. It may have the capabilty to provide downrange velocity and/or time of flight. From what I've heard, the downrange projectile must be supersonic for it to work.
I have extensive experience with the Oehler System '89, have actually used it at 2 miles. Not many have actually begun to use it. My opinion was because it had skyscreens. It now will work with an outside MV source (Garmin, Labradar, Magnetospeed...whatever you trust).
The system is much easier to use now and will provide reliable data - I've run it beside Infinition radar and they gave the same numbers.
These are exciting times to be experimenting with projectile drag.