A more conventional approach is to place a chronograph at the muzzle and another downrange. Firing a bullet through both chronographs will show you the bullet's velocity decay directly. It's simple, but there is a flaw with this approach. To get the most accurate BC measurement, you want to maximize the range between the two chronographs. Positioning them 100 yards apart is not adequate, and even 200 yards is marginal. You want at least 300 yards between the two chronographs. Supposing you have a range where you can do this, it leads to another problem. Careful shooters may be able to "thread the needle" from 300 yards a few times, but eventually the inevitable happens, and the chronograph takes a bullet for science. When that happens, the test is over.