Finding cast boolit BC

[TedH]

I was trying to find the BC for the Saeco 645 bullet, a 480 gr. 45 caliber. You guys pointed me to some online calculators that can get you pretty close, but I went a step further. I set up my chrony at 12 feet, and took 5 shots for an average of 1306 fps. Then I set up the chrony 100 yards from the first chrony position and fired 5 more shots through it. I'd be lying if I said I wasn't more than a little bit nervous shooting the Sharps through my chrono at that range, but they all made it threw cleanly without damage to the chrono. lol Those downrange bullets were traveling 1190 fps by the time they traveled that 100 yards.

I used the calculator on JBM Ballistic to input the velocity data, along with other data such as temperature, humidity, etc. That gave me a BC of 0.391 for the flight of the first 100 yards. I don't know just how accurate that is, but it should be more accurate than a calculator that just uses the weight, length, etc for inputs.

http://www.jbmballistics.com/cgi-bin/jbmbcv-5.1.cgi

[Tom Myers]

When a BC is determined from bullet profile and weight, that is an ESTIMATED value of the ballistic coefficient.
When a BC is determined the way you did it, that is a CALCULATED value of the ballisitic coefficient and is accurate and valid for trajectory calculations under the same conditions and muzzle velocity.