Quote Originally Posted by angrybear View Post
I know it's something to do with the relation of muzzle velocity to down range velocity,
but iirc it also changes depending on the initial MV & the end range.

If you have Chairgun the BC is included for the pellets listed in their database.
The BC values given by Chairgun and other ballistics "apps" or programs are averages. BC values depend on many things and are quite individual per gun. BC's from springers are quite notably different to BC of same pellet from a PCP for instance, reggeed PCP's can give a different BC value depending on the reg pressure and a non-regged PCP has a different BC when high pressure to that when the pressure is dropping, it changes over the fill cycle.
There is part of the Chairgun program that includes an option for calculation of the actual BC of your pellet from your gun, there are various methods but 2 chronos (one near muzzle and one downrange) can be used, or there is an option for measuring the "drop" of a pellet or "time to target". Using these methods will give the real BC from your barrel with that type of pellet.
Using Chairgun select the "toolbox" dropdown, then "calculate and callibrate" then select "Calc BC from" to choose your method.
If using 2 chrono's then obviously you need to check them both at/near muzzle to see that they agree with each other, I have used a Combro on the muzzle and a F1 chrony at 50 yards, I was able to confirm my F1 read only 5-6 FPS slower than the MV recorded by my Combro with the F1 set at 3 feet out from the muzzle with JSB Exact .177 at 782-785fps MV which is close enough (in my opinion).
Obviously if measuring "drop" you will need to use an indoor range and compare group centres seeing as it would be impossible to measure 2 POI's from the same pellet... Unless you have a very fancy high speed video set-up with a scale and two cameras at say 25 and 50 yards so your pellet doesn't have to impact any target to show trajectory difference.
Timing a pellet is a possible option but measuring the time from muzzle to 50 yard target would be best done sonically, remembering that if one microphone is used there is a delay (due to the speed of sound) on the impact noise travelling back to the mic. This could be accounted for by sitting the mic in the middle (i.e. 25 yards) and using a program such as Audacity to sample and analyse the muzzle blast and target impact noise with the delay between timed to the nearest millisec (also filtering out any echos from building structure etc.) The mobile phone "apps" aren't good enough for this in my experience but I haven't actually tried this method with a laptop.