Question for you... How well would shooting through 2 chronies work for getting an accurate BC? Say one at muzzle and another at 300 yds?
In theory this can go very good, in practice, it can go very bad!
The first step would be to set the chrony's up right against each other and shoot thru them to insure they're both reading the same velocity (or the expected 2-3 fps difference). This is key.
Next, make sure you have the accuracy to thread the needle before you try shooting thru the chrono at 300 yards.
Be sure you measure the atmospherics (air temperature, pressure and humidity).
It's very important that you measure the exact
distance between the chrony's; with as little uncertainty as possible (+/- 1 foot is pretty good).
After you've collected the data, you can go to:
JBM - Calculations - Ballistic Coefficient (Velocity)
and plug it in. This will result in a very accurate BC that's corrected for standard atmospheric conditions. The BC will be the average value over 300 yards.
If you're ambitious enough, you can set the downrange chronograph at 100, 200, and 300 yards to get an idea of how much the BC changes with velocity. Another way would be to load your ammo fast and slow and shoot the same distance. This will make it harder to thread the needle.
Here's a link to an article
I wrote about measuring BC's with the two chrno method. The article focuses on dispelling the worry that excessive pitching/yawing motion could affect the measured BC over short distance.
I hope you decide to take this on and share your experiences. Please be careful with that downrange chrono!