Actual Bullet Ballistic Coefficents

[d31tc]

A thread that introduced me to another thing to put on my "list" to buy - the Garmin Xero C1. It seems it has pros and cons compared to a Labradar, one of which is that it apparently doesn't give multiple velocity readings at multiple distances, which can be used to calculate the "actual" Balllistic Coefficient.

I've just used the manufacturer's published B.C. which has worked for my purposes out to 500ish yards. I've read, mostly on gun forums, that some manufacturers MIGHT exaggerate the BC of their bullet for marketing and sales purposes. I don't know if that is the case with any bullets I've used, but I am curious to know what member's experience has been regarding published BC and their own calculated BC using equipment such as a Labradar.

Has anyone created a data library with "accurate" BC data results compared to manufacturer's published data? Is it common for the calculated BC to be significantly different than manufacturer's published data? If so, is it because of the velocity assumed, ballistic coefficient being a function of the drag coefficient, which is a function of the flow velocity, which can change based on muzzle velocity as well as decreasing velocity based on the range of the target?

Hello weeds, I'm getting in.

 

[Thomasss]

Manufacturers generally have laboratory conditions, or they go to company that has "perfect conditions" to test their quality control products.
When they test their stuff they want it to succeed. Actually they want it to excel. Advertising sells. Advertising needs to be a step up from the competition. And we know the buying public wants a super sonic super duper bullet that will perfectly hit the target in a fantastic way with lightning speed and the force of a locomotive. So I won't say they lie. I will say they "test" their product in NASA type conditions. When I hunt, it's under 20 degrees, early dark mornings, target deer are up to 300 yards away. It's snowing, or freezing rain and windy. No one will ever admit their ammo was tested that way. If it was, you will only read about it in a hunting magazine.
I have my own ballistic info. It is limited to the few calibers I shoot and it is usually only given out to those that need some help. I'm on my fifth Chronie. I have used Hornady's Ballistic calculator on the internet and rarely do my numbers match theirs. I just bought my own ballistic calculator and can't wait to get it, so I can calculate my own numbers. They used to say computers: garbage in, garbage out. That's extreme. Rather there are occurrences in any auto system not quite as accurate as I would like to see. So I will start with a Chronie and build the info by hand to my satisfaction.

 

[jmr40]

A bullets BC changes with velocity. It's BC at the muzzle won't be the same as when it is 500 yards downrange. Plus things like temperature and altitude change velocity. I don't think anyone is trying to deceive anyone. They pick one velocity and work the numbers from there.

For the majority of shooters this is close enough. To be honest nothing beats actually shooting at those ranges to verify your bullet drops and fine tune scope adjustments. But using the provided info can usually get you pretty darn close.

 

[mcb]

BC is supposed to be a constant for a given bullet. If you use the correct BC and correct drag model (Gx) then BC does not change with velocity. The drag coefficient does change with velocity and is captured within the Gx drag model (basically an empirically derived table with the drag coefficient values at a particular velocities). The BC is used to scaling all those drag coefficient values to match your bullet. That is the whole point to using BC and the Gx models.

That said modern VLD (Very Low Drag) bullets do not fit the common G1 or G7 drag models very well. Rather than create a new Gx model for these VLD bullets they got lazy and shoe horn it into the older models by splicing segments, each scaled by a different BC, of the older drag models into something that more closely fits the real word data. This is not the way the Gx models were intended to be used although it does work well enough in most cases.

Applied Ballistics has gone to the next step and completely abandoned the Gx drag models and BC and put the actual experimentally measured drag as a function of velocity data for each bullet into the program so rather than scaling a Gx model with a BV to get the drag at a particular velocity they use actual drag data measured for that specific bullet.

 

[Double Naught Spy]

BC is supposed to be a constant for a given bullet. If you use the correct BC and correct drag model then BC does not change with velocity.

How can the ballistic coefficient be the same if in fact drag does in fact change with velocity, which it most certainly does. Many people treat it as a constant, but it is not a constant.


"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."

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

"...a bullet’s ballistic coefficient changes with its velocity."

4.2 Ballistic Coefficient Effects on Bullet Trajectories - Sierra Bullets

www.sierrabullets.com

"The ballistic coefficient changes with both distance and velocity. Few bullet makers acknowledge this..."

Ballistic Coefficients Do Not Exist!

You may actually apply a given value over a given distance, but the BC won't actually be the same over the entire distance as the velocity will most certainly change.

 

[MEHavey]

Dittos above.
I realize my fluid mechanics days are long (long) ago, but drag is a power function of fluid velocity.
(...and since BC is a function of drag . . . .)

 

[mcb]

BC by design is supposed to be a fixed, for each bullet, scaling factor for a velocity dependent drag function (G1, G7, etc) that does change with velocity.



These are the standard projectiles (G1 Top, G7 Bottom) used to create the G1 and G7 drag models. The graphs are the experimentally measured drag coefficient at various velocities for that model bullet shape. The base model bullet has an assumed BC of 1.

If your bullet has a reported BC = X than you take the table of data (for the appropriate model) and divide the coefficient of drag data (for all the velocities) by X resulting in a new table of velocity dependent drag data for your BC = X.

This is what your ballistic software is going when you enter a G1 or G7 BC. It takes the standard G1 or G7 table of drag coefficient data (as a function of velocity) and scales it by the inverse of the entered BC.

The above functions work extremely well if you bullet is the same exact shape and just a scaled up or down of the model. That is pretty rare. But in most cases the resulting scaled drag coefficient table is close enough for many applications. The catch is many of the new really long bullets (VLD) do not closely match the shape of the two models above and thus their specific drag coefficients as a function of velocity have a different shape than what is show above so when you scale the standard Gx function with a BC you get larger errors, usually and at extreme end of the data set and especially near Mach 1. To reduce this error over the most used velocities without creating a new Gx model they have taken to piece wise fitting the actual drag data to the standard Gx models. The puts extra burden on you ballistic calculators as they have to smooth the step transitions between the piece wise BC but its just math and does create a more accurate model for many of these new bullets. Though a new Gx model would be an even better solution. The best solution is what Applies ballistics is doing and simple create a drag coefficient table like the above for each specific bullet and creating a large data base of these bullet specific drag models.

ETA: There a several more model than just G1 and G7 (G1, G2, G5, G6, G7, GI, GL, GS, RA-4, etc) but for what ever reason many ballistic software only has G1 and G7. JBM Ballistics is free online and offers all of the afore mentioned models

 

[InTheSwamp]

In practical terms, it is a constant. You won't find meaningful changes due to velocity for a given projectile. But velocity does affect it.
And yes, manufacturers do exaggerate it.

The Truth About Ballistic Coefficients

Aerodynamic Drag and Gyroscopic Stability

And finally, Wikipedia

 

[InTheSwamp]

Dittos above.
I realize my fluid mechanics days are long (long) ago, but drag is a power function of fluid velocity.
(...and since BC is a function of drag . . . .)

Correct.

 

[d31tc]

Well, I have some good reading material. If I read it 50 times I might start to understand it. If I do, I'll give myself an honorary PhD from my course of unaccredited studies at Wossamotta U.

Interesting that Barnes publishes a G1 BC for a 150gr 7mm TTSX when the bullet shape for a ballistic coefficient seems to be closer to the G7. I'm guessing at the ranges that 90% of shooters use (me included), G1 vs G7 is negligible and most shooting past that modify to match real world results.

For my ranges, I've generally used it to get close and then let my real world results dictate, so close is good enough. Interesting that companies publish, to put it nicely, "optimal" BC's when those that shoot at distances where it starts to really matter, know what the deal is and, I'm guessing, aren't using published BC's to make purchasing decisions. I could be wrong. I'm not one of those people - yet.

 

[Gun-Reck]

BC = Half-Life of Muzzle Energy, in yards (forgetting the decimal point).

Done.

 

[Jim Watson]

You could dig out your Hatcher's Notebook and study Chapter XXIII Exterior Ballistics. Then work it out with pencil and paper. Maybe a slide rule and adding machine.
Me, I'll keep getting by with the streamlined personal computer stuff.

 

[Gun-Reck]

You could dig out your Hatcher's Notebook and study Chapter XXIII Exterior Ballistics. Then work it out with pencil and paper. Maybe a slide rule and adding machine.
Me, I'll keep getting by with the streamlined personal computer stuff.

If it's important, which most likely it isn't?

Chrono and Zero the load for 200 yards, then shoot precision 5-shot groups at 100, and 3 or 400.

Then back the drop data into the Hornady ballistic computer to find what the actual BC is.

 

[d31tc]

If it's important, which most likely it isn't?

Chrono and Zero the load for 200 yards, then shoot precision 5-shot groups at 100, and 3 or 400.

Then back the drop data into the Hornady ballistic computer to find what the actual BC is.

This is what I've generally been doing; and using it for initial shots to get on paper. With my D.O.P.E. card, I probably wouldn't need a ballistic calculator for the ranges I shoot, although it helps.

 

[Jim Watson]

Chrono and Zero the load for 200 yards, then shoot precision 5-shot groups at 100, and 3 or 400.

Then back the drop data into the Hornady ballistic computer to find what the actual BC is.

Many years ago there was an article in Gun Digest about doing it the other way 'round.
'The Trajectory Chronograph'
There then being no consumer Chronographs, it described how to shoot targets at different ranges, measure the "drop" and with that and the published BC calculate by equation and nomograph to get a muzzle velocity.

 

[Sofie007]

A thread that introduced me to another thing to put on my "list" to buy - the Garmin Xero C1. It seems it has pros and cons compared to a Labradar, one of which is that it apparently doesn't give multiple velocity readings at multiple distances, which can be used to calculate the "actual" Balllistic Coefficient.

I've just used the manufacturer's published B.C. which has worked for my purposes out to 500ish yards. I've read, mostly on gun forums, that some manufacturers MIGHT exaggerate the BC of their bullet for marketing and sales purposes. I don't know if that is the case with any bullets I've used, but I am curious to know what member's experience has been regarding published BC and their own calculated BC using equipment such as a Labradar.

Has anyone created a data library with "accurate" BC data results compared to manufacturer's published data? Is it common for the calculated BC to be significantly different than manufacturer's published data? If so, is it because of the velocity assumed, ballistic coefficient being a function of the drag coefficient, which is a function of the flow velocity, which can change based on muzzle velocity as well as decreasing velocity based on the range of the target?

Hello weeds, I'm getting in.

Yes.
Yes.
And 'Yes'

 

[GeoDudeFlorida]

Many years ago there was an article in Gun Digest about doing it the other way 'round.
'The Trajectory Chronograph'
There then being no consumer Chronographs, it described how to shoot targets at different ranges, measure the "drop" and with that and the published BC calculate by equation and nomograph to get a muzzle velocity.

That’s what I learned. Then again back in my day we tended to use mathematics quite a lot, despite the lack of computers.
If you know the atmospheric conditions - temperature and humidity - you can include drag as a variable and calculate velocity at any distance just from the experimental results of drop. The one flaw being gravity is assumed to be a constant.
I haven’t done those numbers for decades. Now I just use a BDC calculator on my phone.

 

[Walkalong]

When I first started PRS I used the BC Berger stated and it worked great until I passed 900ish yards, after which I was higher and higher.

I “trued” the BC at distance using the ballistic program and shots on target, and all has been well since.

Phil from MPA has a good video on truing velocity at 500 and BC at 1000. Works well.

If you love math, it could keep you busy a long time

https://m.youtube.com/results?sp=mAEA&search_query=Mpa+truing+bc

 

[Anchorite]

When I shot benchrest (LV,HV, Sporter) I just learned to read the wind flags.

 

[Walkalong]

That helps, but for long distance you really need the right BC inputed in your ballistic software.

 

[Laphroaig]

Has anyone created a data library with "accurate" BC data results compared to manufacturer's published data?

Bryan Litz has determined BC's independently for many bullets in the JBM Ballistics program.

JBM - Calculations - Trajectory

www.jbmballistics.com

They are denoted in the bullet library by "Litz." The value is given in the output when you run the trajectory calculation. I can't vouch for the accuracy of his numbers, but I use them if they are available for the bullet that I am using. So far, they always have been.

Interesting that Barnes publishes a G1 BC for a 150gr 7mm TTSX when the bullet shape for a ballistic coefficient seems to be closer to the G7. I'm guessing at the ranges that 90% of shooters use (me included), G1 vs G7 is negligible and most shooting past that modify to match real world results.

I've ran trajectory calc's using both G1 and G7 values, and the difference is negligible until you get out past 500 yds. G1 values are always higher than G7 so the manufactures show them to better appeal to the masses.

 

[d31tc]

Many years ago there was an article in Gun Digest about doing it the other way 'round.
'The Trajectory Chronograph'
There then being no consumer Chronographs, it described how to shoot targets at different ranges, measure the "drop" and with that and the published BC calculate by equation and nomograph to get a muzzle velocity.

I would think a similar problem would exist with back calculating velocity, this would assume an accurate published BC. If the BC was optimistic, then so goes the velocity. Fortunately for me, I started this part of my hobby not that many years ago and, as you know, consumer chronographs are pretty reasonably priced nowadays and I could back calculate the BC if it made enough of a difference for me. Now we have laser rangefinders, doppler chronographs, hand held anemometer weather stations, and ballistic calculators on seemingly every device. I sure am lucky they have all of these things I can buy so I'll never miss another shot. Ever.

 

[Chuck R.]

Strelok Pro will also "correct" based on "should hit" VS "did hit" data (it's how we registered "big guns", back in the day)

But what you end up correcting for is hard (impossible??) to differentiate due to the variables involved.

IMHO, the delta in conditions probably plays a larger roll, both atmospheric exterior, and the effects of temp on MV than having the errors in your BC.

 

[Anchorite]

That helps, but for long distance you really need the right BC inputed in your ballistic software.

True.

 

[d31tc]

Strelok Pro will also "correct" based on "should hit" VS "did hit" data (it's how we registered "big guns", back in the day)

But what you end up correcting for is hard (impossible??) to differentiate due to the variables involved.

IMHO, the delta in conditions probably plays a larger roll, both atmospheric exterior, and the effects of temp on MV than having the errors in your BC.

I don't disagree about the other variables involved. For the ranges I shoot, at most I see a difference of 0.1 mil at 500 between the ballistic calculator and my shot group. I'm probably the biggest variable.