Quote Originally Posted by rancidtom View Post
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.

BC's Should not vary hugely from different guns etc. Most of the differences reported are down to people simply not carrying out the tests properly, using different methods to actually calculate the BC with different reference drag laws, not taking into account the atmospheric conditions such as wind speed and direction and so on. There can be differences between springers and PCP's due to pellet distortion, but even this should not be large. BC's should be fairly constant for any given pellet if the test is carried out correctly and the correct reference drag law has been used. This is the whole point of using BC's.

The most accurate method is by measuring the time of flight and muzzle velocity, followed by the method using two chronos and lastly measuring drop. Don't try the last one, there are too many other factors affecting the drop. It is vital that for all methods you measure the atmospheric conditions if you want a BC that can be used under all conditions, and use a calculation method like ChairGun where you can correct for the atmospheric conditions.

If you only want the BC to use on the day of your test, then you can get away with not bothering with the atmospheric conditions unless they are rapidly changing. When you use your BC in any trajectory calculations, the atmospheric conditions should be much the same, so the BC will work. On a different occasion when conditions are different, you will have to do another test as your BC will not be correct.

In the end, it all depends what you want the BC for and just how accurate you want your calculations to be.