Kestrel 5700 elite V1.23 applying waaay too much spin drift?

DennisT

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Jun 28, 2018
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#1
Hi,

I have just recieved my brand new Kestrel 5700 Elite with Link, and have run into the following problem I hope you can help me solve.
It seems that the Kestrel is adding waaay too much spin drift, and I can't find where I have put "garbage in" to get "garbage out" - please help!

I have added the folowing gun model for the Lapua Naturalis (I am yet to measure the MV, so the 798 m/s is a guesstimate):
MV: 798 m/s
DM: G1
BC: 0.201
BW: 9.1 gm
BD: 6.71 mm (bullet lenght is automatically calculated from BC and BD)
ZH: 0.00 mm
ZO: 0.00 mm
RT: 1:21.00 cm R (one revolution pr. 210mm on my 56 cm Schultz & Larsen barrel)

BH: 5.15 cm
ZR: 100 m

When having set Tgt to 500m and Wind to "None" (W1: 0 m/s W2: 0 m/s), I am getting the following firing solution:

E: 7.18 Umil
W: 2.57 Lmil

The elevation seems plausible, but not the windage! - when I go to the Kestrel ballistic menu, to see the contributors to the windage, I see that spin drift is adding 2.51 Lmil and Coriolis 0.06.

When I turn off Spin drift, I get the following firing solution:

E: 7.18 Umil
W: 0.06 Lmil

As I don't believe the magnitude of spin drift to be true, I have run the same parameters in StrelokPro and got E: 7.2 Umil (Great!) and W: 0.1 Lmil, which is plausible.

What am I doing wrong in the Kestrel?

----
The first thing I did after having unpacked the Kestrel was to update it through the iPhone APP, so I am running:

Model: 5700AL
SN: 2269015
FW: 1.23
BOOT: 1.01
BLE: 0.06
BAL: 44
HW: Rev 11B

Best regards,
Dennis
 
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pell1203

Full Member
Dec 27, 2007
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Bellevue, WA
#3
DennisT, value this opinion for what you paid for it... only offering it because you have gotten no other suggestions.

I'll preface all by stating, I neither own a Kestrel nor do I use any flavor of Applied Ballistics software anywhere. Not familiar with it at all!

However, since your issue appears to be windage related, and perhaps more specifically related to spindrift, I'd double check you've provided the Kestrel with the bullet weight, length, and barrel twist data in the correct units and magnitude expected by the AB software.

For example, I noticed in your original post that bullet weight had been provided as 9.1 grams (gm). Oftentimes bullet weight info is expected to be entered in grains, which in your case would be more like 140 grains (gr). If you entered barrel twist in cm and it was expected in mm, this could also throw calcs way off.

Not likely this is an issue but confirm an unexpected rifle cant value has not been introduced anywhere into your calculations. A canted rifle will significantly alter the windage values required to stay on target and your software may be trying to correct for this unintended input.

Hopefully others with more directly applicable experience will soon hop onboard and help you figure it all out. Good luck!
 
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DennisT

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Jun 28, 2018
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#4
Hi pell1203,

thank you for your observations!

As I understand it, spin drift is dependent on the following parameters, that I have checked and double checked in the Kestrel:

- Muzzle velocity: 798 m/s
- BC: 0.201 G1
- Caliper: 6.71mm
- Time of bullet flight: 1.076sec (for the 500m shot)
- Bullet length: 11.10mm (maybe a bit short, but it is calculated automatically by the Kestrel, as mentioned)
- Bullet weight: 9.1gm or 140gr (value 9.1 changes automatically to 140 when I change the Unit from "gm" to "gr" in the Kestrel, so that should be OK)
- Rifle twist: 1:21.00 cm R (value 21.00 changes automatically to 8.27 when I change the Unit from "cm" to "in" in the Kestrel, so that should be OK)

- Station Pressure: 1013.1 mb (Measured by the Kestrel, and seem plausible)
- Humidity: 34% (Measured by the Kestrel, and seem plausible)
- Temperature: 23 deg C (Measured by the Kestrel, and seem plausible)
- Density altitude: 329 m (Measured by the Kestrel) <-- Isn't calibrated though, since I am only 92m above sea level

I don't believe the Kestrel can take rifle cant into consideration.

are there some other factors I am overlooking?
 
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DennisT

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Jun 28, 2018
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#6
OK. I have now made the drift calculation myself (see attached picture), and it yield a drift of 2,9MIL, when using the parameters stated above.
I am pretty sure of the BC, Caliber, Weight, MV, Barrel twist and temp, so I tried to vary the altitude and bullet length.

Changing the altitude doesn't change the SG and hereby the drift much, but changing the bullet length does!

I think its the 11.10mm bullet length automatically calculated by the Kestrel that is the problem. When I change it manually to 32.79mm (which is the actual length of the Lapua Naturalis bullet according to JBM ballistics "Bullet length list"), the spin drift is reduced to 0.19 L MIL!

The questions are now:
Am I right?
Is it the actual bullet length I should physically measure with a caliper, and feed into the Kestrel?
Why is the Kestrel automatically calculating the bullet to be 11.10mm long?!

Drift calc.JPG
 
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DennisT

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Jun 28, 2018
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#8
All this metric nonsense is giving me a headache 🤪 but isn’t your G1 number too low? I thought it was listed by Lapua as .329.
Haha - MIL and Metric is a pretty smart combination ;) 1 MIL = 10 cm @ 100m :love: Inches and yards on the other hand is giving me a headache, since I come from Denmark and most info on LR shooting is coming from the US (y)

Anyways. According to the Lapua homepage, the G1 BC for the Naturalis 6,5mm (.264) is 0.201 (and yes, that is absolutely horrible!)
 
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Jul 18, 2013
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#9
Hmmm lapua does list that G1, but Euro optic, Midway USA, and pretty much everybody else lists the higher number. Maybe two different bullets? Anyway good luck with sorting it out, I am out of ideas!
 

DennisT

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Jun 28, 2018
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#10
Hmmm lapua does list that G1, but Euro optic, Midway USA, and pretty much everybody else lists the higher number. Maybe two different bullets? Anyway good luck with sorting it out, I am out of ideas!
OK! - strange :unsure: I tend to believe the value supplied by Lapua though.

Also, I believe that I have sorted it out. See Post #6, but the questions still stand:

- Am I right?
- Is it the actual bullet length I should physically measure with a caliper, and feed into the Kestrel?
- Why is the Kestrel automatically calculating the bullet to be 11.10mm long?! What formula lies behind.
 

pell1203

Full Member
Dec 27, 2007
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#11
DennisT, you may have arrived at your answer on your own. The bullet length may not be an actually computed item produced by the software. It most likely is just directly retrieved via direct lookup out of the Applied Ballistics bullet database library loaded in the latest software revision. If the value stored in the database is wrong, then your results follow the garbage in-garbage out paradigm.

For grins, try selecting another bullet from the library and rerun the calculations to see if your answer is correct or reasonable with the new bullet combo. Then to confirm your suspicions, scale down and modify the bullet length directly in your Kestrel to approximately 1/3 of the actual measured value to determine if you are able to introduce a similar error.

If so, then I would not be the least surprised if the lookup length for your Lapua bullet has simply been erroneously entered into the bullet database.
 
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DennisT

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Jun 28, 2018
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#13
Hi DennisT
I took your profile information and for a 500 m shot, no wind I get E 6.68 and W 0.30. I suggest you create another gun profile in your app and transfer it to the Kestrel and see if you get the same results. Initially, I would look for an entry error.
Hi Kestrel Ballistics,

the gun profiles I have uploaded is shown in the screenshot below - do you see any entry error? - it is not possible to enter the bullet length in the app, only directly in the Kestrel.

Image-1.jpg
I have transferred the "Test" gun to the Kestrel, as you suggest, and now it looks like this (at SP: 1007.5 mb, RH:34% and Dalt: 435m):

IMG-5156.JPG IMG-5157.JPG IMG-5158.JPG IMG-5159.JPG IMG-5160.JPG IMG-5161.JPG IMG-5162.JPG IMG-5163.JPG IMG-5164.JPG

Does that seem right to you? I guess you have other environmentals, than I currently have, so it seem plausible since it is not way off to what you are getting - right?

I believe the wrong bullet length that was automatically calculated by the Kestrel for the Naturalis profile was just a glitch, since it is now calculated to a more correct value of 28.16mm (even though it is not equal to the actual measured bullet length of 32.79mm).

What is best length to use. The value calculated automatically by the Kestrel, or the actual measured bullet length from tip to end as a manual input?

Thank you for the support.

NB: After having imported the "Test" gun, I did turn the Kestrel from manual bullet length calculation to automatic, before it arrived at 28.16mm, so maybe that is what did the trick.
 
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Jul 12, 2014
60
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Alaskan bush
#14
I've struggled with this exact problem before, too. For me, used a smartphone interface and had troubles between my manual charts / calculations and what the kestrel said (kestrel was wrong). One of the first things I did was scour the bullet profile, and sure enough, the length of bullets I was fighting with the Kestrel was way off, as it was automatically calculated.

Corrected it with manual entry of actual measurement and voila. IMHO you should NOT rely on the automatic calculation of bullet length!! Enter the actual measured bullet length.

Strange that your phone doesn't have a manual entry point, because on my phone there is a space for manual entry of bullet length, which I had to find and correct for the same reasons as you. Now it matches the charts almost perfectly.

Looking at the math, the last element that goes into calculating the b.c. is the form factor (b.c. = lbs/in^2, where form factor is in the denominator with diameter^2), and it appears as though the form factor may be affected by length. Consider the relative aerodynamic drag of a long streamlined pointed cylinder vs a short round ball (like the old musketballs, for example). That difference going into the b.c. will affect the drag curves downrange as well as time of flight, which would affect the resultant spin drift. I can only assume that the Kestrel's equations include some of this, and it is spitting out erroneous stuff simply because of that math error related to incorrect bullet length.

Found this from berger and it sure seems to indicate length is a consideration in form factor, which goes on to affect b.c.
Berger bullets form factor discussion

At least that's what I tell myself, lol. But most of that is just speculation...
 
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DennisT

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Jun 28, 2018
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#15
Hi Plidenbrock,

thank you very much for your input. Great to know that I'm not the only one who has experienced this!

Actually, I don't understand why this flaw in the Kestrel has not been discussed before (I wasn't able to find any info on Google or the forums), since this must be a common error generated very often, when relying on the Kestrels automatic calculation of bullet length. If the windage wasn't so obviously wrong, as it was in my instance, heck, I wouldn't have given this any attention at all!

Since it is not possible to manually define the bullet length in the current Kestrel software for the iPhone, but only afterwards in the Kestrel itself, my guess is, that several users of the Kestrel is not defining bullet length, because it doesn't seem important. Why search for an entry the Kestrel by default automatically calculates for you, right?

This is not what I expected when buying a top of the line ballistic solver that has gone through numerous iterations to arrive where it is today... Why indicate that the actual length of the bullet is not an important entry for the firing solution? Why not have users to measure bullet length (or look it up, as I eventually did), exactly like users are expected to measure MV and BH?

I am happy that I found this flaw before utilizing the Kestrel at all. That has saved me a lot of time and frustration at the shooting range and at competitions.
 
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DennisT

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Jun 28, 2018
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#17
I like when posters come to their own conclusion and avoid bad advice. I found this same issue when I used the auto bullet length on AB Analitics so I always double check bullet length.
Well, thanks. I hope that other Kestrel users can use this learning and avoid the pitfall of wrong bullet length when calculating spin drift. I also hope that Kestrel will take this into consideration when doing the next software upgrade for the app and for the Kestrel itself.
 

Kestrel Ballistics

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Feb 2, 2018
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kestrelinstruments.com
#19
Well, thanks. I hope that other Kestrel users can use this learning and avoid the pitfall of wrong bullet length when calculating spin drift. I also hope that Kestrel will take this into consideration when doing the next software upgrade for the app and for the Kestrel itself.
Dennis - I have gone through several tests trying to duplicate the issue you saw with bullet length but was not able to do it.

If any users out there have had similar issues and can help us duplicate the steps to reproduce, we will certainly fix this.

One other note, there is a setting in the Kestrel LiNK Ballistics app that called Bullet Length Auto Calculate. You can set this to NO and then in the profile you will have the option to enter the bullet length manually. So you don't always have to enter this on the Kestrel itself. Click on the menu icon in the top right and go to Settings and you should be able to change this item for the future.
 
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DennisT

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Jun 28, 2018
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#20
Dennis - I have gone through several tests trying to duplicate the issue you saw with bullet length but was not able to do it.

If any users out there have had similar issues and can help us duplicate the steps to reproduce, we will certainly fix this.

One other note, there is a setting in the Kestrel LiNK Ballistics app that called Bullet Length Auto Calculate. You can set this to NO and then in the profile you will have the option to enter the bullet length manually. So you don't always have to enter this on the Kestrel itself. Click on the menu icon in the top right and go to Settings and you should be able to change this item for the future.
Hi Kestrel Ballistics,

Thanks for the tip regarding turning off automatic bullet length calculation in the Kestrel app - that is good that I don't have to manually correct it every time the gun profiles are renewed on the Kestrel.
When I turned off the auto calc. in the app, then the bullet automatically calculated lengths showed for the 3 gun profiles I have created. Common for the profiles was, that the bullet length was "1.110 cm" (before I changed them to the right value) - considering that I have transferred the gun profiles from the app, and not created a profile directly on the Kestrel, I guess that the App has introduced the error.

When creating a Profile, I have not selected a bullet from the bullet library, but done manual input. That should be possible for you to duplicate, right?

Fresh installation of the Kestrel app (here BL Auto Calculate is set to ON) --> Set all settings to Metric --> New profile --> Input data manually (0.67cm, 140gr, 0.201G1, 798m/s, 100m, 5.15cm, 0.00cm, 0.00cm, 21.00cm-R, mil, mil, leave blank) --> Transfer data to Kestrel --> Set auto cal to OFF both in the App and in the Kestrel, and see what bullet lenght is shown. When doing this, 1.110cm should show, since that is exactly what I have done.
 
#21
Hi Kestrel Ballistics,

Thanks for the tip regarding turning off automatic bullet length calculation in the Kestrel app - that is good that I don't have to manually correct it every time the gun profiles are renewed on the Kestrel.
When I turned off the auto calc. in the app, then the bullet automatically calculated lengths showed for the 3 gun profiles I have created. Common for the profiles was, that the bullet length was "1.110 cm" (before I changed them to the right value) - considering that I have transferred the gun profiles from the app, and not created a profile directly on the Kestrel, I guess that the App has introduced the error.

When creating a Profile, I have not selected a bullet from the bullet library, but done manual input. That should be possible for you to duplicate, right?

Fresh installation of the Kestrel app (here BL Auto Calculate is set to ON) --> Set all settings to Metric --> New profile --> Input data manually (0.67cm, 140gr, 0.201G1, 798m/s, 100m, 5.15cm, 0.00cm, 0.00cm, 21.00cm-R, mil, mil, leave blank) --> Transfer data to Kestrel --> Set auto cal to OFF both in the App and in the Kestrel, and see what bullet lenght is shown. When doing this, 1.110cm should show, since that is exactly what I have done.
DennisT - Are you using Android or iOS?
 
#22
DennisT - Are you using Android or iOS?
Nevermind. It looks like it is an iOS app issue as I was able to duplicate it with iOS, just not Android. We will look into fixing. Thanks for your help in identifying this. 2.817 cm = 1.1 inches. So it looks like there is a conversion issue there where it is pulling imperial value but using metric units.
 
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