First of all, BCs should be measured with either the velocity loss or the time of flight technique. Using bullet drop is prone to many errors because bullet drop is sensitive to many uncertainties and confounding factors that do not exist at all or have much smaller effect on velocity loss and time of flight techniques.
Secondly, anyone who cares about BC accuracy and effects on retained velocity, wind drift, and drop should read Bryan Litz's book.
We've published four papers on BC issues and acoustic measurement techniques:
http://arxiv.org/ftp/physics/papers/0601/0601102.pdf
http://arxiv.org/ftp/arxiv/papers/0812/0812.4752.pdf
http://arxiv.org/ftp/arxiv/papers/0705/0705.0391.pdf
http://arxiv.org/ftp/arxiv/papers/0705/0705.0389.pdf
Even though we've validated an acoustic technique with simultaneous use of near and far chronographs, the two chronograph method is more accurate and highly recommended where possible, because small errors in various measurements (near velocity, distance, temperature, air pressure, etc.) have smaller impact on BC determinations with two chronographs.
In our two chronograph method, we verify that the two chronographs give the expected velocity drop (1-4 fps) when placed a few feet apart before we extend the distance to 300 or 600 feet.
We have seen significant variances in BCs between different rifles, and even within the same rifle before and after polishing the bore. Lots of factors effect bullet drag. In the absence of a BC measurement in a specific rifle, Bryan Litz's numbers are usually the best available estimate, but I strongly prefer to measure the BC in my rifle using two chronographs spaced by 300-600 feet. A single chronograph and our acoustic technique for your specific rifle would also be preferable to just going with any published number. In contrast, you're probably better off going with a published number than with a BC estimate determined from drop.
Michael Courtney
Thanks for your contribution Michael, ive read your papers and like the ideas presented - however i can see some problems within your methods, and if done differently, would improve the accuracy/resolution of the accoustic method of determining BC - same goes for the 2 chronograph method.
First some of the problems i have with both methods;
1. The testing is not done at far enough distances to provide a good enough AVERAGE of the BC in a long range flight regime - if your only measuring across a distance of 300 or 600 feet, then you can only calculate an average BC in the high velocity part of the flight regime. As we know, the BC decays with decreasing velocity, so we really want to know the average BC over much longer ranges such as 1000yds. So- why not build a steel shield for the second chrony @ 1000yds so we dont smash it, and shoot for averages there in order to get better resolution from much greater velocity decay and better "average BC"? - many dont bother, because its a PITA to setup the second chronograph way out there with sheilds and making sure its actually working before you waste all you ammo etc etc...
Now the same thinking applies to the accoustic method, longer ranges, longer Time of Flight, better resolution, and a more complete flight regime in which to average the BC over. The problem with the method presented in your paper, is with being able to record the sounds accurately - which i guess is why you dont do it at further ranges... So i offer this simple improvement to your method, originally suggested to me by Bryan Litz...
Take the computer and microphone into the pits (where you score and mark the targets protected behind a mound at the dangerous end of the rifle range) of a sanctioned 1000yd rifle range - so you know the distances are accurate. You will also need a 2 way radio at both ends, and have the shooter key the transmitter before he pulls the trigger and shoots thru a chronograph (or 2 chronographs front to back even better) At the recieving end, you get the muzzle blast come thru the 2 way, then some time later the sonic crack of the bullet as it passes overhead. Allowing approx 0.001 sec for each foot the microphone is from the bullet hole in the target, and the transmitter is from the shooters muzzle at the other end, you get a very good Time of Flight measurement over 1000yds -which is usually more than a 1 second ToF depending on the caliber. Reverse calculate the BC using a G7 calculation if your using a VLD type bullet, to match your measured time of flight @ 1000yds and your chronographed velocities. Repeat this 10 times if you wish, in as quick or slow succession as you like, then take a mean or median average, upto you.
Again tho, this is only as good as the operators attention to detail - but it does eliminate much of the errors associated with aiming errors, zero errors, drop errors, scope tracking errors etc - they are completely gone.
The second issue i have is with trusting chronographs. You may have verified the chronies are accurate to each other when placed front to back, but they may be in different light when you shift one out to 300ft - 600ft, and making sure they are perpendicular to the bullet flight etc all contain error... Additionally whilst they may be reading the same, is it an accurate reading or simply that they reading the same inaccurate velocity in those light conditions? Either way, the more distance you have, the better resolution you get and these velocity errors become less influential on the outcome of the calculated average BC...
