Re: How do I calculate the ballistic coefficient?
Kind of the same as Brent said and I agree that you need very accurate equipment to come up with "true" BC but....
For practical purposes even if you do not have a really accurate chronograph you can get very repeatable results by shooting over whatever you have and back calculating the results based upon actual bullet drop. I use Exbal for this purpose. I get a hundred yard zero and then shoot it out at 500 yards and adjust the scope until centered. Then I adjust the BC figure until the program calculates the scope correction to equal the actual correction needed using the average velocity the cheapo Chrony displays. Then I test at 3,5,8 and 1000 and I have very repeatable drops based upon these values in three rifles. If I change a bullet, I have to do it again. Likewise if I were to change the Chrony, I believe I would have to readjust.
In my case I use .550 for the Nosler 200gr Accubond and I have found a value of .600 to work well for a 220 SMK.
This probably means that the chrony is giving readings that are higher than "true" or that the Bullet BC changes based upon velocity in flight. I do not know, I just know that it works. So to me the only reason to need a very accurate chronograph to calculate BC would be by a bullet manufacture so the published data was as correct as it could be for consumer comparison. But for the average Joe, poking holes in paper while shooting over a $60 Chrony works quite well provided you shoot enough at the extended ranges to be confident in "your system".
PS: Don't use the BC values that I have listed as I feel the adjusted BC is proportional to the accuracy of speed of the chronograph that you are using. That said, I use the manufactures listed BC as a starting point and go from there.