The REALLY, real- real BC

[AlwaysLastPlace]

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

 

[JungleShooter]

Frustrated in Fenix,

do not despair.
Help is around the corner — when the US wakes up again.
Because not everybody is
Sleepless in Seattle.

Yours truly,
Positive in Peru



PS: On a not so Positive note:
Our world isn't ideal — that's whey there are certain fluctuations in BC.
Part of that is the margin of error of each chrony. Some have a higher margin — and make BC values less precise.

● Do you have any wind (which will mess with BC!)?

● What BC calculator do you use? Does it allow to take atmospheric conditions (esp. your elevation!) in to account?

● Have you compared the readings of each chrony — are they nearly the same shooting through them one right after the other?
In a series of shots in this setup does the difference between the two readings fluctuate much ➔ then one chrony (or both) are imprecise.
No need for a $500 Labradar. Two $70 chronies will do the trick.

I'm going to put Positive to bed now. Tomorrow I have a lot of killing to do at a new permission....

 

[Ballisticboy]

Most things are covered above. The first thing I would add is that the range you are using is a bit short. The shorter the range, the more the errors in the chronographs will affect your answers as the errors will be a larger percentage of what you are trying to measure, which is the loss of speed over 25 yards. Secondly, do not use the old constant Cd calculators which are on many websites. Use a modern calculator with a full range of reference drag laws for pellets and slugs. For round nose diabolo pellets, use the GA or GA2 reference drag models.

 

[EquivalentDelta]

It’s important to consider that BC increases linearly as speed decreases in the subsonic range.

 

[stovepipe]

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

 

[ChrisT]

numbers are great but the only thing that counts is meat on the table. Everything in in mother nature tilts the numbers. hard evidence is what shows on target or in the brisket.

 

[Ballisticboy]

I see the usual statements being made as facts again. All the above about BC changing with speed and multiple BC values being needed for modelling are based on the use of the wrong reference drag law to start with. If the correct reference drag law is used, then a single value will be all that is needed at all speeds, this is the basic fundamental idea of the entire BC system of calculation. All these myths come about by the use of the constant reference Cd in the early BC calculations, that is what gave Bob Sterne's graphs showing large changes in BC with speed. Bob now uses the pellet specific drag laws for his BC work, or the slug drag laws for slugs. The reference drag laws now available for the different pellet types reduce the need for multiple BC values, particularly at usual pellet ranges. For long ranges, there may be small changes in the BC value. If large changes in BC are required, then you are using the wrong reference drag law. There are now GA, GA2 and WC0 drag laws available for pellets and SLG0, SLG1 and RA4 for slugs. G1 should never be used for any airgun projectiles, particularly at higher speeds.

 

[stovepipe]

I see the usual statements being made as facts again. All the above about BC changing with speed and multiple BC values being needed for modelling are based on the use of the wrong reference drag law to start with. If the correct reference drag law is used, then a single value will be all that is needed at all speeds, this is the basic fundamental idea of the entire BC system of calculation.

Your statement "If the correct reference drag law is used, then a single value will be all that is needed at all speeds, this is the basic fundamental idea of the entire BC system of calculation." is entirely correct!!!!

You bash everyone that says things like "the BC changes as the velocity changes", or "multiple BCs need to be used".

You point out that there are now NEW drag laws. Yes, NEW drag laws keep coming out and have been for decades.

New drag laws keep coming out because many projectiles do not fit into the existing models and, consequently, there is no single BC that works for those projectiles -- yet.

Us little average guys with cellphone apps or inexpensive desktop programs that have a less-than-complete availability of drag models in our little low-budget world have to make due with drag models that are not custom-made for every projectile we use.

So given the lack of drag laws/models available compared to the huge number of projectiles available, it really isn't fair of you to pretend to not understand when one of us little guys says something like "the BC changes during flight", or "I needed to use multiple BCs to get the ballistic program to match my DOPE.". We are expressing our experiences and observations the best way we can.

stovepipe

 

[Ballisticboy]

You point out that there are now NEW drag laws. Yes, NEW drag laws keep coming out and have been for decades.

New drag laws keep coming out because many projectiles do not fit into the existing models and, consequently, there is no single BC that works for those projectiles -- yet.

Us little average guys with cellphone apps or inexpensive desktop programs that have a less-than-complete availability of drag models in our little low-budget world have to make due with drag models that are not custom-made for every projectile we use.
stovepipe

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.

 

[Scotchmo]

G1 is commonly used for airgun slugs because most know that a higher BC value is better. And G1 gives the highest values, so, right(not) or wrong, that’s what gets used all too often.

 

[stovepipe]

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.

Thanks for pointing out the "newer" drag models. Many of these I've never heard of. It's been a long time since I tried to get a ballistics program to agree with my dope. None of the existing drag laws in my ballistics programs at the time were a good match, so I gave up. I guess it's time to try again using the newer drag models.

stovepipe

 

[stovepipe]

G1 is commonly used for airgun slugs because most know that a higher BC value is better. And G1 gives the highest values, so, right(not) or wrong, that’s what gets used all too often.

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

 

[Scotchmo]

Thanks for pointing out the "newer" drag models. Many of these I've never heard of. It's been a long time since I tried to get a ballistics program to agree with my dope. None of the existing drag laws in my ballistics programs at the time were a good match, so I gave up. I guess it's time to try again using the newer drag models.

stovepipe

Try RA4. It’s in most ballistic apps. If you’re running higher BC airgun slugs in the 900s or up to 1100fps, RA4 works well. For slugs that are 2 to 3 calibers long, it works especially well.

Shorter, lower BC slugs below 900fps can work OK with G1.

 

[stovepipe]

Try RA4. It’s in most ballistic apps. If you’re running higher BC airgun slugs in the 900s or up to 1100fps, RA4 works well. For slugs that are 2 to 3 calibers long, it works especially well.

Shorter, lower BC slugs below 900fps can work OK with G1.

Thanks Scotchmo. I've been shooting NSA 54.5 grain slugs at 961 fps with my fx maverick 30 cal sniper for a while. I had to email NSA to find that they used the G1 profile to get the BC of 0.12. I have strelok pro and I just now checked and it does have the RA4 profile. The 54.5 gr slugs are kind of shorties -- not even 2 x the caliber in length. I need to have a good shooting session off of a stable platform (I rarely have that option) to gather fresh dope, then I'll play with the RA4 profile. Thanks again.

stovepipe

 

[Scotchmo]

Thanks Scotchmo. I've been shooting NSA 54.5 grain slugs at 961 fps with my fx maverick 30 cal sniper for a while. I had to email NSA to find that they used the G1 profile to get the BC of 0.12. I have strelok pro and I just now checked and it does have the RA4 profile. The 54.5 gr slugs are kind of shorties -- not even 2 x the caliber in length. I need to have a good shooting session off of a stable platform (I rarely have that option) to gather fresh dope, then I'll play with the RA4 profile. Thanks again.

stovepipe

You can convert from the 0.120 G1 to get a 0.097 RA4. But always best to get your own numbers. How far are you shooting? If it’s 100yds and under with a 961fps muzzle velocity, either might work well enough.

JBM - Calculations - Drag Function Conversion

jbmballistics.com

 

[stovepipe]

You can convert from the 0.120 G1 to get a 0.097 RA4. But always best to get your own numbers. How far are you shooting? If it’s 100yds and under with a 961fps muzzle velocity, either might work well enough.

JBM - Calculations - Drag Function Conversion

jbmballistics.com

View attachment 496180

Scotchmo, this is awesome. I didn't know a conversion could be done. I initially printed out the G1-based holdovers (sight-in at 55 yards) from 10 to 300 yards at 2-yard intervals. I'll do the same with this RA4 conversion and compare them. Most of my shooting is targets within 100 yards (I like trying to hit tiny targets), but my scope setup will let me dial up to 300 yards without needing to hold over (in theory). My friend also shoots this rifle and he shoots over 200 yards sometimes at ground squirrels.

stovepipe

 

[Scotchmo]

Scotchmo, this is awesome. I didn't know a conversion could be done. I initially printed out the G1-based holdovers (sight-in at 55 yards) from 10 to 300 yards at 2-yard intervals. I'll do the same with this RA4 conversion and compare them. Most of my shooting is targets within 100 yards (I like trying to hit tiny targets), but my scope setup will let me dial up to 300 yards without needing to hold over (in theory). My friend also shoots this rifle and he shoots over 200 yards sometimes at ground squirrels.

stovepipe

The conversion works under the assumption that the G1 BC is correct at the 961fps. If G1 is not a good match and the 0.120 BC was obtained at some other velocity, then ???. It’s still a good place to start. Best way is with a Labradar to verify..

 

[JungleShooter]

New drag laws [...] 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.

Thank you, Miles, for using your professional training and decades of experience in ballistics to do what very few airgunners could:
Giving us new and better drag laws!

Matthias


PS:
The drag laws Miles mentions above as "having been created" — really have been created by him!
And they are available to all in the (free) ballistic suite GPC Ballistics, for Android, Mac, Windows, Linux, here:

GPC Ballistics Applications for Windows, MacOS and Linux

A few ballistics applications for Windows, macOS and Linux written in the PureBasic programming language.

gpc.fotosoft.co.uk

 

[AlwaysLastPlace]

It’s important to consider that BC increases linearly as speed decreases in the subsonic range.

I didn't realize bc was changing. That makes a lot make more sense. Thank you.

 

[AlwaysLastPlace]

Frustrated in Fenix,

do not despair.
Help is around the corner — when the US wakes up again.
Because not everybody is
Sleepless in Seattle.

Yours truly,
Positive in Peru



PS: On a not so Positive note:
Our world isn't ideal — that's whey there are certain fluctuations in BC.
Part of that is the margin of error of each chrony. Some have a higher margin — and make BC values less precise.

● Do you have any wind (which will mess with BC!)?

● What BC calculator do you use? Does it allow to take atmospheric conditions (esp. your elevation!) in to account?

● Have you compared the readings of each chrony — are they nearly the same shooting through them one right after the other?
In a series of shots in this setup does the difference between the two readings fluctuate much ➔ then one chrony (or both) are imprecise.
No need for a $500 Labradar. Two $70 chronies will do the trick.

I'm going to put Positive to bed now. Tomorrow I have a lot of killing to do at a new permission....

I have done very few of these things in the way you described. I will keep at it.
Thanks.