Hi - I am new to air gun. Can someone point me to the right direction on how to calculate: 1) ft lb energy based on grain and FPS and 2) BC?
Ft-Lb energy and BC
- Thread starter Dotdotdot
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FPE = 1/2mass x velocity^2
For that basic equation, velocity is in FPS, but mass is in slugs (not grains).
For grains and FPS we need to convert grains to slugs.
FPE = 1/2 x grains/(225218 grains/slug) x FPS^2
or
FPE = grains/450436 x FPS^2
BC is more complicated as it requires the amount of velocity degradation over a known distance at known atmospheric conditions, and each projectile shape uses a specific drag model to assign a BC. The simple BC calculator referenced by Moderator3 is most likely for the GA pellet drag model at standard atmospheric conditions, so its suitability will depend on wether you are using pellets only at near sea level.
For that basic equation, velocity is in FPS, but mass is in slugs (not grains).
For grains and FPS we need to convert grains to slugs.
FPE = 1/2 x grains/(225218 grains/slug) x FPS^2
or
FPE = grains/450436 x FPS^2
BC is more complicated as it requires the amount of velocity degradation over a known distance at known atmospheric conditions, and each projectile shape uses a specific drag model to assign a BC. The simple BC calculator referenced by Moderator3 is most likely for the GA pellet drag model at standard atmospheric conditions, so its suitability will depend on wether you are using pellets only at near sea level.
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I think it actually uses the constant drag coefficient method, which is inaccurate at high muzzle velocities. It is much better to use a BC calculator with a selection of reference drag laws. There are plenty available but make sure they have pellet reference drag laws, not G1, if you have a rifle with a muzzle velocity greater than 600-700 ft/sec.
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Thanks all! It seems like it’s rather difficult to calculate the actual BC. However, if I hold the drag law / coefficient constant, I can still have a good comparison of the performance of the pellet / slug.
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I look at BC as something best measured instead of calculated. I use values published by hard air magazine or, if they are not available, data from the ammo provider. What I really want to know is the bc for that projectile from my gun but they can't provide that. If you want it bad enough to risk a chronograph you can measure velocity at the target a known distance away from the muzzle and calculate the bc for that projectile in your gun (on that day and at the atmospheric conditions that existed at the time of the shot). Long way of saying bc is a lot tougher to determine than fpe.
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Having some shift in Air Density be it from Humidity, Temperature or Elevation will change the BC value of a projectile. So while one may get close in an approximate for any given projectile and speed shot, it's BC value kinda floats around with atmospheric changes.
Case in point is what I'm about to deal with this weekend. Shooting an FT match in the high desert of Nevada @ 4500 ft.
My baseline in BC at sea level runs @ .038, At my home of @ 1500ft about .040, where headed to shoot will realize .044 or better.
This based off @ 70* and 40% humidity .. tho not etched in stone.
This is a three fold change ... Dryer air, decrease in air density due to higher elevation and warmer temperature.
It's fun, like chasing a cat
Case in point is what I'm about to deal with this weekend. Shooting an FT match in the high desert of Nevada @ 4500 ft.
My baseline in BC at sea level runs @ .038, At my home of @ 1500ft about .040, where headed to shoot will realize .044 or better.
This based off @ 70* and 40% humidity .. tho not etched in stone.
This is a three fold change ... Dryer air, decrease in air density due to higher elevation and warmer temperature.
It's fun, like chasing a cat
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If your drag model is a good match to the projectile, and you account for the environmental conditions (mostly air density), your BC should be constant.Having some shift in Air Density be it from Humidity, Temperature or Elevation will change the BC value of a projectile. So while one may get close in an approximate for any given projectile and speed shot, it's BC value kinda floats around with atmospheric changes.
Case in point is what I'm about to deal with this weekend. Shooting an FT match in the high desert of Nevada @ 4500 ft.
My baseline in BC at sea level runs @ .038, At my home of @ 1500ft about .040, where headed to shoot will realize .044 or better.
This based off @ 70* and 40% humidity .. tho not etched in stone.
This is a three fold change ... Dryer air, decrease in air density due to higher elevation and warmer temperature.
It's fun, like chasing a cat
Most of the popular ballistic apps let you input the environmental conditions before calculating the BC. If you try to calculate the BC without using the correct environmental factors, you will get a different number.
I have a similar elevation change situation coming up for RMAC. All of my development is done at sea level. That’s where I get my BC data. The RMAC range is at about 4900ft. I use the same BC at both locations along with their current environmental factors. Not that important for benchrest, but extremely important for the long range magnum slug matches.
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Drag models and complex math ... outside this simpletons wheel house
Tho seem to do ok in spite.
Scott, I applaud your efforts in all your pursuits of shooting perfection ...
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If you are using a ballistic app, there is a good chance that it will do all the math for you. For pellets below about 950fps, the GA drag model is fine. Your phone can give you the local environmental conditions either using a weather app to download current conditions or some phones have the sensors built in (iPhone).Drag models and complex math ... outside this simpletons wheel house
Tho seem to do ok in spite.
Scott, I applaud your efforts in all your pursuits of shooting perfection ...
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