Testing the b c of a 177 pellet

[Magpie palmer]

Does anyone know now to test the b c of a 177 pellet?

Thanks

[angrybear]

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.

[B&W FOX]

There is a book called Air Gun from trigger to muzzle by G V Cardew that will answer your questions .
Expect to pay 20 - 30 pounds for a copy.

[rancidtom]

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.

[ballisticboy]

There is a book called Air Gun from trigger to muzzle by G V Cardew that will answer your questions .
Expect to pay 20 - 30 pounds for a copy.

This book does not tell you how to obtain BC. The follow-up book "Air Gun from trigger to Target" dealt a little with BCs, but again the information is old and not relevant these days. I know, I helped Gerald to write it.

[ballisticboy]

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.

[angrybear]

Thanks chaps, reminded me why I'm happy with chairguns figure

[Retnuhtibbar]

Strelock pro is how I found the BC of my pellets, Chairgun never got it right for me. The drop was never right at greater distances, but with Strelock you can input the correction to what it estimated and it recalculates,

[Neil54]

Chronograph at muzzle and another in front of a target at a known distance.
Then you can calculate bc.

There is a bit of maths in the middle.

[GPConway]

You could try my EasyBC application - available for Windoze/MacOS/Linux and Android.
The applications can use any of the methods (∆POI, ∆T and ∆V) mentioned by Ballisticboy above depending of the facilities at hand.
They also have access to the various drag laws appropriate to airgun pellets and slugs.

The Android version is available from the Google Play Store (*** here ***) and the desktop versions from my own domain (*** here ***)

George

[Born Again]

Years ago when I cared about such things I measured the velocity at the muzzle and the drop at several distances, then fiddled with the BC in Chairgun until the predictions matched what I was getting. There's probably a technical reason why I was wrong, but it did what I needed - enabled me to print out a useable trajectory and windage table.

[pjinoz]

Does anyone know now to test the b c of a 177 pellet?

Thanks

Yes you can use 2 chronographs try NOT to hit the second one! I use LabRadar to determine the bc’s of my home made .25 cal slugs and have only tried one pellet;33.95 gr. JSB which computed to a bc of 0.046 using the GA function. Slugs need the G1 function ( can use JBM program), pellets ChairGun. But as others have said bc will be influenced by the velocity range and to a lesser degree gun/barrel. LabRadar will give you velocities over a range of distances and you can choose what velocity band you to measure bc. Of course the 2 chrono method could be used but with more setup time involved. On the other hand Hard Air Magazine website has many bc’s in their ballistic coefficient section and your pellet type may be included.
Cheers

[zephyr]

Yes you can use 2 chronographs try NOT to hit the second one!

Y’know, it’s never occurred to me try to hit the chrono! With CTC groups of circa 5mm at 26yds, unless using a Combro at the target, it’s not a problem to miss it.

[ballisticboy]

Yes you can use 2 chronographs try NOT to hit the second one! I use LabRadar to determine the bc’s of my home made .25 cal slugs and have only tried one pellet;33.95 gr. JSB which computed to a bc of 0.046 using the GA function. Slugs need the G1 function ( can use JBM program), pellets ChairGun. But as others have said bc will be influenced by the velocity range and to a lesser degree gun/barrel. LabRadar will give you velocities over a range of distances and you can choose what velocity band you to measure bc. Of course the 2 chrono method could be used but with more setup time involved. On the other hand Hard Air Magazine website has many bc’s in their ballistic coefficient section and your pellet type may be included.
Cheers

The G1 drag law is not suitable for slugs, the shape of a slugs drag law is completely different to the G1 law. This is why many slugs seem to have higher BC's at high speeds. I know many manufacturers use the G1 law, but all that shows is that they don't know any better.

If the correct drag law is used, there should be very little variation in BC with speed. Most of the claimed variations are due to poor experimental technique, not correcting for atmospheric conditions and using the wrong calculators with different reference drag laws.