[EDIT] CLOSED [was HELP NEEDED]: project to calculate BC using Labradar track data

[ptosis]

EDIT 2021-04-24: the calculator is available, check this thread -- https://www.snipershide.com/shootin...7-bc-calculator-from-labradar-tracks.7074446/

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Dear happy owners of Labradar devices,

As a new happy owner of a Labradar device, I could not resist playing with the track files that Labradar writes to the SD card for every recorded shot. The data is absolutely amazing (I just wish emission power was not so limited for general public use).

I bought a Labradar for hassle-free measures of V0 (and it works marvels). Now I realise it may be feasible to do what I did not think was possible: calculate ballistic coefficients from Labradar track files.

Why would we need that?

- Sometimes newbies come to the range, knowing only what's written on the ammo package. We were all newbies once, we ought to help them out.
- Sometimes (for inexpensive ammo) the BC of a given batch is easily 5% or more off the data published on the official site.
- Some manufacturers just don't publish BC data (someone please tell RUAG Ammotech "welcome to the 3rd millennium")
- Sometimes you just ask yourself "can I reliably hit this 800m target with an AK74 using Soviet block surplus ammo?" (yes, I am a pervert and I enjoy it)
etc.

Preliminary tests look very promising; I am getting within 1-2% of reference values, which is ok for all practical purposes. This said, I am rather limited in the number of different projectiles I shoot, and in the possibilities to run more comprehensive tests until at least March 2021.

Hence, this is a CALL FOR HELP

If you have Labradar records of bullet tracks (the TRK folder on the SD card, "Shot XXXX Track.csv" files), for Lapua or Hornady bullets (so that I can compare to Doppler-measured Cd curves), recorded in known atmosphere, I would very much appreciate if you could share them.

In return,

If all goes well, I will make available a calculator producing a BC value from Labradar track files, for all to use under a free and open source license.

If all does not go well, I will produce some text describing the error sources and limitations of the approach, so that others don't waste time where I would have already wasted.

What is needed:

1. Atmosphere: pressure, temperature and (if available) humidity. This is absolutely paramount, without proper atmo data, the rest of it is useless. An indication of the method used to measure atmo (e.g. "Kestrel 4500") would be very helpful to estimate error margins.
2. The bullet used. Lapua or Hornady ONLY, please; these guys are the only ones to produce publicly available Doppler-measured curves for their products – the "golden standard" reference to compare to.
3. The actual track data, a ZIP archive of the whole "TRK" folder (or the whole Series folder) – at least 10 shots of the same bullet recorded in the same atmosphere. The speeds do not have to be consistent or uniform, e.g. loads testing/development data is just fine, as long as projectile and atmosphere are the same.
4. If known, the rifling twist (according to a paper published by Hornady, a rifling twist, which is very different from what is considered "average" for the calibre/speed, could explain a BC deviation of up to 2%).
5. The model of your Labradar: (a) full juice US/CA/AU or (b) castrated EU.

You can send it all by PM here on snipershide (it should be possible to attach a file to a message)
or by e-mail to "guns [at] <my snipershide user name> [dot] ch"
or simply post here as attachment (as I just did as an example for 10 shots of a Swiss ordnance GP90 from a Stgw90; yes, the Swiss army bulk infantry ammo has a SD < 3 m/s)


Thanks and cheers,
P.

 

[LastShot300]

Please keep me posted, I'm really curious about since the manufacturer states very clearly that the unit is not meant for measuring BCs at least with a certain degree of accuracy and reliability. I know some people got some useful data but again, good only for short to medium ranges, and obvioulsy not good enough for LR work.

 

[ptosis]

Please keep me posted, I'm really curious about since the manufacturer states very clearly that the unit is not meant for measuring BCs at least with a certain degree of accuracy and reliability. I know some people got some useful data but again, good only for short to medium ranges, and obvioulsy not good enough for LR work.

Yep, will do.

I also was under the impression that 50-80 m or so of trajectory was vastly insufficient to get a reliable BC figure. Yet, when I crunched some numbers around the tracks data, I got surprisingly close to robust verified figures.

Now I'd like to see if that was just a coincidence for the specific projectiles I tested, and generally understand the limits of the approach and the error margin.

 

[AFancyPenguin]

Nerd! Just kidding. I love stuff like this. I dont shoot either brand bullet much, but following to see how it goes. Good luck.

 

[stewarmy01]

The tracking distance is too limited to correlate to any BC’s at distance, where BC really matters. It could give you an idea of your MAX BC, but as velocity falls off over distance, so does your BC. Sort of in line with Sierra showing BC’s over different velocity ranges. You could get some data, sure, but nothing worth spending much time on. Using an external ballistics software and calibrating your muzzle velocity and atmospheric conditions would be the most realistic way of correlating actual BC’s.

 

[LastShot300]

Yep, will do.

I also was under the impression that 50-80 m or so of trajectory was vastly insufficient to get a reliable BC figure. Yet, when I crunched some numbers around the tracks data, I got surprisingly close to robust verified figures.

Now I'd like to see if that was just a coincidence for the specific projectiles I tested, and generally understand the limits of the approach and the error margin.

Hope you all the best in your experiments, but AFAIK Labradar is not the tool for the job, except when you need a starting point to further refine an initial BC, in such scenario it's for sure a handy help.

 

[ptosis]

The tracking distance is too limited to correlate to any BC’s at distance, where BC really matters. It could give you an idea of your MAX BC, but as velocity falls off over distance, so does your BC. Sort of in line with Sierra showing BC’s over different velocity ranges. You could get some data, sure, but nothing worth spending much time on. Using an external ballistics software and calibrating your muzzle velocity and atmospheric conditions would be the most realistic way of correlating actual BC’s.

Judging by the references you make to Sierra bullets data, you seem to reason in terms of G1 model (this is sooo 20th century), in which the BC of real-world bullets is indeed very velocity-dependent.

The G7 model, however, corresponds to modern spitzer boat tails bullets much better, and a single BC figure usually fits quite nicely the whole Mach 1.4 - 3 range. This is the model I will be using.

As a side note, I am not sure I understand your last sentence. MV and atmo are not to be calibrated, they are to be measured. How would you use a ballistic solver to "correlate" a BC from there?

 

[ptosis]

Nerd! Just kidding. I love stuff like this. I dont shoot either brand bullet much, but following to see how it goes. Good luck.

YEAH! A nerd, a geek, and proud of it
Thanks for the encouragement!

 

[stewarmy01]

This is why I said “sort of in line” just to illustrate that BC’s drop as velocity decreases. My last sentence assumes you put the factory “alleged” BC into the program. Once you enter your measured MV and atmospheric conditions, your waterline at distance would assist you in “calibrating” the actual BC of the bullets at your current condition (MV, atmospherics, etc). There is also the statistical error of the chronograph in this equation that could lead to a BC that isn’t actual because your input MV is wrong by 1-2%. Sorry for any confusion.

 

[ptosis]

Hope you all the best in your experiments, but AFAIK Labradar is not the tool for the job, except when you need a starting point to further refine an initial BC, in such scenario it's for sure a handy help.

Thanks, and you're right -- Labradar is definitely not built for the task.
But when I run it against the GP90 ordnance ammo, and I get .167, while the figure derived from the Swiss Army data is .166 -- I think there may be something. Especially since the typical use case would be a situation where I have absolutely no clue of the BC, and, as you say, I need something to get started.

 

[stewarmy01]

Thanks, and you're right -- Labradar is definitely not built for the task.
But when I run it against the GP90 ordnance ammo, and I get .167, while the figure derived from the Swiss Army data is .166 -- I think there may be something. Especially since the typical use case would be a situation where I have absolutely no clue of the BC, and, as you say, I need something to get started.

I agree, that something your are finding is MAX BC.

 

[LastShot300]

Thanks, and you're right -- Labradar is definitely not built for the task.
But when I run it against the GP90 ordnance ammo, and I get .167, while the figure derived from the Swiss Army data is .166 -- I think there may be something. Especially since the typical use case would be a situation where I have absolutely no clue of the BC, and, as you say, I need something to get started.

Of course, it's a great aid when needing a starting point, indeed a nice technical approach to do this instead of the usual "fudging", having said this it's for sure a very nice experiment to conduct. Please keep us posted and all the luck

 

[ptosis]

Of course, it's a great aid when needing a starting point, indeed a nice technical approach to do this instead of the usual "fudging", having said this it's for sure a very nice experiment to conduct. Please keep us posted and all the luck

... and to end the day on a rather more gentlemanly exchange --

Thanks! I will post the progress here (if and when I get some data to crunch).

 

[LastShot300]

... and to end the day on a rather more gentlemanly exchange --

Thanks! I will post the progress here (if and when I get some data to crunch).

Al

... and to end the day on a rather more gentlemanly exchange --

Thanks! I will post the progress here (if and when I get some data to crunch).

You know, we were always gentlemen, no excuses here even when facing opposite views, otherwise where the fun is if we all think the same?

 

[stewarmy01]

Good luck ptosis, I am interested in seeing what you find. Wasn’t trying to be an ass if I came off that way!

 

[Steel head]

Way too short of distance to get an accurate BC in my opinion

 

[ptosis]

Good luck ptosis, I am interested in seeing what you find. Wasn’t trying to be an ass if I came off that way!

Thanks! (And don't worry, you did not come across as agressive or anything.)

 

[ptosis]

Way too short of distance to get an accurate BC in my opinion

The distance should be ok; the trace file contains enough data to approximate the curve rather precisely, and averaging over multiple shots should level out the errors.
What I am more worried about is that the device takes its readings over the very beginning of the trajectory, where effects of intermediary ballistics / bullets "going to rest" might still play an important role. I understand the BC over this segment may be lower than optimal (but I don't know how significant the effect would be).
But for now it's just my speculations; actual data should make it clear.

 

[ptosis]

Just finished coding the prototype calculator. So far the results are very encouraging.

Next step (if all goes well): the meeting scheduled in 10 days with Her Majesty Reality.
7 guns, 5 calibres, 8 different projectiles, and, time permitting, total ~300 rounds.

In the meantime, if somebody suddenly decides to help the Shoestring Ballistic Science effort, please do share your tracks, and let the results surprise you. Early access to the prototype will be provided to contributors.

(As a side-rant: I can't believe that between 161K registered members, and probably hundreds of Labradar devices in their ownership, nobody cares to keep digital archives, or to share them. So much for the "shooters community".)

 

[LastShot300]

Looking forward to your results

 

[ptosis]

Here's something:

Labrabaco: a G7 BC calculator from Labradar tracks

Yes, it is possible. Yes, it does work. I have put together some code to calculate G7 ballistic coefficients from Labradar tracks. The tool is available here: https://bc.geladen.ch/labrabaco/labrabaco.html It all runs in your browser, there is nothing to install, and it's free and open source...

www.snipershide.com