The REALLY, real- real BC

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AlwaysLastPlace

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Sep 6, 2024
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Gilbert, AZ
Edit- thanks everyone for a lot of answers. Before I posted this I did not know enough to simply ask the question any better.
So here is what (or how) I think I should have asked it:
Why does the drag law I am being told, or recommended, to use in certain apps differ?
Why does the bc on the tin not match what the calculator says it will do AND not match what I see in real life?
I have a ton of stuff to go try out now and I don't feel stuck so thanks.

P.S.
Some people are super smart and they like showing off and that can come across as talking down. But I grew up on the less fun end of a one-way range so it's pretty hard to hurt my feelings. I'm new to improving at this level, not 10 years old.
People can be any combination of wrong, right, nice or a big turd about it. It's all paying for education and now I've got homework or else I just wasted everyone's time with a less than optimal question that ya'll kindly took the time to try to answer.
I can tell you all that there are a LOT fewer and less angry know it alls in this world than in real guns, 3 gun especially. Some folks maybe should be nicer and other people maybe need to let more attitudes slide.


Guys and gals I have been doing my homework which means I have tried figuring it out on my own before bothering you fine folks.
I have a chrony built into my barrel and one at 25 yards.
I am using JSB Exact 44.75gr, a well calibrated mg scale and a brand of micrometer for head size that is so German I'd get kicked out of Disneyland (again) for saying it out loud. What am I doing wrong?
I'm getting bcs from low .03s to mid- high .04s.
I've read some things that are way too complex that I don't have equipment for, radar and such, but we have been figuring bc on paper forever.
What do I do? I intended to go through all my JSB weights and play on all the ballistics calcs to scratch my autism itch for the day but I never got past the JSB exact 44.75.
Frustrated in Fenix
 
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Ballisticboy said:
All of the drag laws mentioned above can be found in free, openly available software for phones and laptops. It costs you nothing. The problems arise when fire arm apps, which do not have the correct drag laws, are used for pellets.

New drag laws for pellets have not been coming out for decades. GA was the first one to replace the previous constant Cd methods which had been used up till then. Follow on ones such as GA2 and WC0 have been created in the last few years in an effort to help average guys to use the free apps readily available. The same with the slug specific ones SLG0 and SLG1. RA4 is for rimfire, but seems to work OK for slugs at most velocities. None of the drag laws will be perfect, but for 99% of applications they will be an improvement on what was available before. You do not need purpose drag laws for every pellet design, and there is very little software available which can use purpose drag laws if you had them.

Obviously for some people we have just been wasting our time.
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I have found them for free as you said but I am at the beginning of learning to use them.
What apps do I have? All the free ones.
If I were to pay for one PC or Android what would it be? But I'd prefer one that I could grow into rather than one that I already have to know everything to use it including solving navier-stokes before I can really use it.
 
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stovepipe said:
Yes, and what's worse is that they print the BC on the box or in the description of the projectile they're selling and they typically never specify what drag model was used.

stovepipe
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Is it just me or is jsb being too conservative? I don't have 30 + inches of drop at a 100 yards. But if I use the .03 whatever number JSB says that is what shows up on the calc. Oh wait maybe I should just use their number and change only the drag law (a term I just learned) to back into what drag law they used? That could keep me busy for a while! I'm not really looking for the answer just a direction to head in learning that seems not to be a waste of time. Which is totally subjective.
 
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stovepipe said:
In this youtube video, Keith Gibson gives an overview of how he uses the Strelok Pro ballistics program. The important thing to get from this video is that he has to use MULTIPLE ballistic coefficients in order to get the program to closely match his real-world D.O.P.E. (data on previous engagement).

The commonly-used external ballistics models (GA, G1, G7, etc.) are clunky general-purpose models that will RARELY be an exact match for a given projectile. Even if one of these models is a close match for a given projectile, the model will only be valid for a limited velocity range. Outside of this velocity range, the model will "fall apart".

Your D.O.P.E. comes first -- you took the shot, you took notes, it's real (or at least it was at the time). Trying to get a ballistics program to agree with your notes can be time-consuming, but it CAN be done.


stovepipe
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Are you aware of any way to get Strelok anymore?
 
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This is interesting. I have figured out how to take raw shot data from LabRadar and all of the downrange velocity data and clean it up. (If you have a LR, it uses very simple math to give you downrange velocity. The radar data past 50y gets a bit choppy and their math increase the effect of that error).

So with smoothed and sensible downrange velocity data I’m looking forward to comparing it high quality ballistic models. Maybe, for once, the ballistic model, the LabRadar data, and actual holdovers will agree!
 
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dgeesaman said:
This is interesting. I have figured out how to take raw shot data from LabRadar and all of the downrange velocity data and clean it up. (If you have a LR, it uses very simple math to give you downrange velocity. The radar data past 50y gets a bit choppy and their math increase the effect of that error).

So with smoothed and sensible downrange velocity data I’m looking forward to comparing it high quality ballistic models. Maybe, for once, the ballistic model, the LabRadar data, and actual holdovers will agree!
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Be careful how you smooth the data. The answer you get will often be distorted by the smoothing method used, to the extent that any drag curves you get can be completely the wrong shape. The only reason I know this is because I had to spend years analysing thousands of radar tracks and, at first, sometimes getting the wrong answers, not really what you want when you are producing fire control data. Many were obviously wrong, some demanding new laws of aerodynamics to explain them if they were correct (I even had one which required accelerating bullets).

The best method I found in the end was to take the unsmoothed data, carry out the analysis, and then smooth the final results. At the furthest ranges, you will probably end up having to do it by eye, as it gets more than the smoothing routines can deal with.
 
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AlwaysLastPlace said:
Is it just me or is jsb being too conservative? I don't have 30 + inches of drop at a 100 yards. But if I use the .03 whatever number JSB says that is what shows up on the calc. Oh wait maybe I should just use their number and change only the drag law (a term I just learned) to back into what drag law they used? That could keep me busy for a while! I'm not really looking for the answer just a direction to head in learning that seems not to be a waste of time. Which is totally subjective.
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You might be mixing up POI with drop. At 100yds, 30inches of drop is about right for a subsonic pellet with a 0.03 BC.

Once sighted in at say 25 or 30yds, you have already set a significant amount of drop compensation. So you’ll only need another 12moa (approximately) of additional correction. That 12moa is not your total drop.

You probably have a time of flight of around 0.4 seconds. 30” is about how far something falls in that brief period of time.
 
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Scotchmo said:
You might be mixing up POI with drop. At 100yds, 30inches of drop is about right for a subsonic pellet with a 0.03 BC.

Once sighted in at say 25 or 30yds, you have already set a significant amount of drop compensation. So you’ll only need another 12moa (approximately) of additional correction. That 12moa is not your total drop.

You probably have a time of flight of around 0.4 seconds. 30” is about how far something falls in that brief period of time.
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Gravity, makes our projectiles and balls droop given enough time. :ROFLMAO:


-Matt
 
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Multiple-BC use is very valid over long distance shooting, it's not an opinion, rather fact, no drag model I know of perfects this, however most ranges airguns are used, its negligible, but as people push further and further, Mutli-BC should not be left in the dust or forgotten.


Also worth noting, behind the scenes for ballistic / trajectory programs calculate an adjusted bc which takes into account your environmental conditions such as altitude and temperature, but they do not calculate new bc's for drops in velocity during the time of flight, which is up to the user to input with multiple bc's. It's why some manufacturers (not pellet) will list multiple bc's for different speeds, so you can use these in your calculations.

So, often times, if your having to fudge your bc, its more often than not your environmental factors are not inputted correctly (possibly due to deviance from where the manufacturer tested their listed bc), or your shooting at a very different speed than the manufacturer did for their testing. Once you determine your bc, a good ballistic program shouldn't need any more changes to that based on where you zero, provided they compensate for the enviromental changes and you input them correctly.

-Matt
 
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Stubbers said:
Multiple-BC use is very valid over long distance shooting, it's not an opinion, rather fact, no drag model I know of perfects this, however most ranges airguns are used, its negligible, but as people push further and further, Mutli-BC should not be left in the dust or forgotten.


Also worth noting, behind the scenes for ballistic / trajectory programs calculate an adjusted bc which takes into account your environmental conditions such as altitude and temperature, but they do not calculate new bc's for drops in velocity during the time of flight, which is up to the user to input with multiple bc's. It's why some manufacturers (not pellet) will list multiple bc's for different speeds, so you can use these in your calculations.

So, often times, if your having to fudge your bc, its more often than not your environmental factors are not inputted correctly (possibly due to deviance from where the manufacturer tested their listed bc), or your shooting at a very different speed than the manufacturer did for their testing. Once you determine your bc, a good ballistic program shouldn't need any more changes to that based on where you zero, provided they compensate for the enviromental changes and you input them correctly.

-Matt
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When I was using G1 for my .257 slugs, I used the Strelok multi-bc function and it worked well. Later, I switched to RA4 in Strelok and a single BC value worked close enough so I did not even bother with multi-bc.

If your drag model is a poor match for your projectile, multi-bc is a workable bandaid. If you use multi-bc, the drag model does not have to match well at all. So use whichever drag model you want. But you will usually have to get the BC values yourself for multiple velocities, and that’s more work.
 
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Scotchmo said:
When I was using G1 for my .257 slugs, I used the Strelok multi-bc function and it worked well. Later, I switched to RA4 in Strelok and a single BC value worked close enough so I did not even bother with multi-bc.

If your drag model is a poor match for your projectile, multi-bc is a workable bandaid. If you use multi-bc, the drag model does not have to match well at all. So use whichever drag model you want. But you will usually have to get the BC values yourself for multiple velocities, and that’s more work.
Click to expand...

Multi-BC are objectively useful based on subjective experience, but there is no ballistic coefficient model that completely rules out the need entirely for everyone, objectively, however a good ballistic model could try to do it for you.

Add to that, the lower the bc of a particular projectile, the more it would benefit from multi-bc inputs in your ballistic program, due to the increase in velocity change between muzzle and target.

I think often times, people adjust their bc to get real world acceptable dopes at longer ranges, basically averaging what would be the use of 2 bc's, ie: .045 + .047 = .046, and then calling it good, without knowing, which then throws off their close range inputs. This is the band-aid you speak of. YMMV.

-Matt
 
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The use of multi BC values was the reason purpose drag laws were introduced for fire control systems 60 years ago, because so many BC values were needed that it became pointless. Now the small arms industry has decided that it is the latest innovation, with bullet makers producing purpose drag laws. It is only a matter of time before they become the newest thing for airguns as well, particularly for slugs at long ranges. Currently though, there are no purpose drag laws for pellets available to the public, and you would have to doubt the ability of many of the manufacturers to produce them. Shooters can produce them themselves if they have the equipment and the data needed, but it is not easy.

It is up to the user to decide if a single BC, multiple BC's or purpose drag laws are needed for the accuracy they want. I personally use purpose drag laws for my work, mainly because that is what I was using at work and the software I have does not use BC.
 
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Ballisticboy said:
The use of multi BC values was the reason purpose drag laws were introduced for fire control systems 60 years ago, because so many BC values were needed that it became pointless. Now the small arms industry has decided that it is the latest innovation, with bullet makers producing purpose drag laws. It is only a matter of time before they become the newest thing for airguns as well, particularly for slugs at long ranges. Currently though, there are no purpose drag laws for pellets available to the public, and you would have to doubt the ability of many of the manufacturers to produce them. Shooters can produce them themselves if they have the equipment and the data needed, but it is not easy.

It is up to the user to decide if a single BC, multiple BC's or purpose drag laws are needed for the accuracy they want. I personally use purpose drag laws for my work, mainly because that is what I was using at work and the software I have does not use BC.
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I'd love to see your newer formulas / models publicized, but until they are, I cannot integrate them into my software, and they aren't available with Sterlok.


Co-efficient definition:

In Physics: a number that is constant for a given substance, body, or process under certain specified conditions, serving as a measure of one of its properties

Therefore its just semantically illogical to state BC is supposed to be a constant, without adding 'under specified conditions', no co-efficient is constant under all conditions.


en.wikipedia.org

Ballistic coefficient - Wikipedia

en.wikipedia.org en.wikipedia.org

The transient nature of bullet ballistic coefficients​

Variations in BC claims for exactly the same projectiles can be explained by differences in the ambient air density used to compute specific values or differing range-speed measurements on which the stated G1 BC averages are based. Also, the BC changes during a projectile's flight, and stated BCs are always averages for particular range-speed regimes. Further explanation about the variable nature of a projectile's G1 BC during flight can be found at the external ballistics article. The external ballistics article implies that knowing how a BC was determined is almost as important as knowing the stated BC value itself.

For the precise establishment of BCs (or perhaps the scientifically better expressed drag coefficients), Doppler radar-measurements are required. The normal shooting or aerodynamics enthusiast, however, has no access to such expensive professional measurement devices. Weibel 1000e or Infinition BR-1001 Doppler radars are used by governments, professional ballisticians, defense forces, and a few ammunition manufacturers to obtain exact real-world data on the flight behavior of projectiles of interest.

-Matt
 
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Wikipedia is not the best reference for anything to do with external ballistics, in fact a lot of it is complete rubbish. For instance, the first sentence in your quote from Wiki, BC, if derived properly, does not vary with air density as it should be calculated at standard atmospheric conditions which of course do not vary. Any calculations using the BC should include atmospheric conditions. It is all standard ballistic calculation methodology, which the Wikipedia author obviously does not know. It is because of things like the Wiki articles and a lot of what you read in magazines and on internet forums that there is so much basic misunderstanding of aeroballistics.
 
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Ballisticboy said:
Wikipedia is not the best reference for anything to do with external ballistics, in fact a lot of it is complete rubbish. For instance, the first sentence in your quote from Wiki, BC, if derived properly, does not vary with air density as it should be calculated at standard atmospheric conditions which of course do not vary. Any calculations using the BC should include atmospheric conditions. It is all standard ballistic calculation methodology, which the Wikipedia author obviously does not know. It is because of things like the Wiki articles and a lot of what you read in magazines and on internet forums that there is so much basic misunderstanding of aeroballistics.
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I'd love the internet forums and wikipedia to be absolved of any inaccuracies, perhaps you should re-write the Wikipedia definition of Ballistic Coefficient? This is an honest statement, not an attempt to troll, I'd love for it to all be corrected. You seem to be the right guy to do it, anyone can edit the article. It would be awesome if you would publish your new GA2 formulations and models here, for transparency and for the good of anyone interested in learning.

Aside from that..most external ballistic models will take into account the change of the conditions (air density and velocity), thus adjusting the bc within the model to represent the real world data being presented, such as the Pesja model, which uses a retardation co-efficient for any given slope constant factor if velocity data points are known.

The Doppler radar measurements disagree for a constant BC below...

toajirbx7u3a1 - Copy - Copy - Copy (2).jpg


"The initial rise in the BC value is attributed to a projectile's always present yaw and precession out of the bore. The test results were obtained from many shots not just a single shot. The bullet was assigned 1.062 for its BC number by the bullet's manufacturer Lost River Ballistic Technologies."

Does the BC change if I shoot a projectile at 150 fps versus 500 fps, 900 or 1500 fps, or 2500 fps? At Sea level, or in 30,000 ft? If so, its not constant.

-Matt
 
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Is Hornady wrong too?

www.hornady.com

Ballistic Coefficient - Hornady Manufacturing, Inc

A-Tip®, ELD® Match, and ELD-X® Ballistic Coefficient Values Hornady originally published “800 yard”* average Ballistic Coefficient (BC) values for ELD-X and ELD Match bullets. This was done to provide the most usable BC from a trajectory prediction st...
www.hornady.com www.hornady.com

"As Hornady moves the industry to drag coefficient based trajectory calculations for these types of projectiles, the Ballistic Coefficient is becoming somewhat irrelevant. Exceptions are for those still using BC based trajectory calculators or when using BC as rating criteria for bullet performance.

The Science Behind It​


Ballistic Coefficient (BC) values can, and usually do change in value with changes in velocity. Most bullets exhibit a lowering BC as velocity slows. The extent of how much a BC will change depends on each unique bullet shape. When comparing BCs of different bullets, it is important to use an apples-to-apples approach.


To do this, bullets should be compared at a given Mach number (e.g. Mach 1 = 1116.48 fps @ ICAO Standard Atmosphere). Mach number is the velocity of the bullet divided by the speed of sound. If a Mach number is unavailable, velocity can be used if the BCs are corrected to Standard Atmosphere which is fairly typical practice within the industry."

-Matt
 
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G1 gives us something to use for comparison, even if it’s not a good match to every projectile we want to compare. Slug specific drag models won’t work for comparisons.

To calculate a BC, we need environmental conditions (mostly barometric pressure), V1, V2, and distance between them. In general changes to environmental conditions don’t affect the BC as long as the current conditions are applied. Environmental conditions affect drag.

I like it when manufacturers give a G1 BC value, but if it is not a good match at all velocities, they should give the velocity range where that BC applies, and maybe even give G1 BCs at different velocities. If there is a good match drag model (RA4 or G7 for instance), they should give that BC as well.

G1 gives us a familiar value that we can compare.
 
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