Thread: Large MV variation from trueing

Keen to pick the brains of some who may really know the weeds better. I have recently finished a load for my new 260 shooting the 142 Nosler Accubond Long Range projectile. For all my loads and testing throughout, the labradar consistently gave me 2700+ fps which I was happy with as my speed goal for the load.

Today I trued using the Kestrel 5700 with custom drag model etc. As a result MV was trued at 2574fps. I confirmed this true at another intermediate range and it was bang on. My zero was on and all else in the kestrel was correct. Even cross checked to my old ballistic app. Accuracy was mint so happy there.

Where I am getting at is how do you end up with such a large MV variation from a tool like a labradar? I’ve trued a few rifles now and some have been on with the labradar and others have required up to a 40-60 fps adjustment but never have I needed to adjust this much before. It’s accurate and on target so that is good but it does have me wondering how the heck the speed is so far out and if I need to re think my load as that is much slower then I would settle for.

Thoughts from those who know their stuff?

The ABLRs used to have severely inflated bc values potentially that? I have a box that claims some unbelievable numbers. Litz values about 80-90% of claimed values.

Originally Posted by Stocky

The ABLRs used to have severely inflated bc values potentially that? I have a box that claims some unbelievable numbers. Litz values about 2/3rds claimed values.

Yup the custom drag model should have that sorted. The BCs on the kestrel are much more realistic to noslers original data if you choose those options from the bullet library too.

I just had a quick Google and found alot of people having to drop bc values to around 580-600 g1 or 291ish g7 ie way lower than even the new doppler results. Stability has a huge effect on the ABLRs and they are not overly stable even in a 1 in 8 twist.

Originally Posted by stagstalker

Yup the custom drag model should have that sorted. The BCs on the kestrel are much more realistic to noslers original data if you choose those options from the bullet library too.

Hmm I'd still be looking at the BCs if velocity is about where you think it should be for the load. As the lad radars are pretty bloody accurate.

Have you checked the scope height and all those simple things?

Have you tried running the data in another program like strelok etc?

Kestrels G7 BC is 0.293

If you have put every thing into the kestrel correctly and used the cdm ignore the labradar as truing is the truth.
I trust my kestrel way over any chronograph.
Does the load data match either the high speed of the chrony or the low speed of the kestrel?
Next question

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Sounds like you are doing most things right and I agree that it is a large MV delta from a CAL MV exercise when one has used a CDM. There is the possibility that you are unlucky and have a barrel that significantly degrades the bullet's BC compared to others but lets work through a common fault checklist first:

1. Are you 100% confident that the distance you CAL MV'd at was correct?
2. Did you update your "environment" prior to conducting CAL MV?
3. How many rounds are you using to get your mean point of impact for your CAL MV?
4. What is your MV SD/ES from your LabRadar?
5. How many rounds does the new barrel have through it?

Feel free to flick me a PM, I can give yo my phone number and we can work through it over the phone.

Originally Posted by JT_NZ

Sounds like you are doing most things right and I agree that it is a large MV delta from a CAL MV exercise when one has used a CDM. There is the possibility that you are unlucky and have a barrel that significantly degrades the bullet's BC compared to others but lets work through a common fault checklist first:

1. Are you 100% confident that the distance you CAL MV'd at was correct?
2. Did you update your "environment" prior to conducting CAL MV?
3. How many rounds are you using to get your mean point of impact for your CAL MV?
4. What is your MV SD/ES from your LabRadar?
5. How many rounds does the new barrel have through it?

Feel free to flick me a PM, I can give yo my phone number and we can work through it over the phone.

@JT_NZ

1. The one thing I definitely didn't do by the book was the range. I shot it at 657 yards rather then the 10% back from transonic which is about 1020 yards for this rifle. The reason I did this is because its a hunting rifle I wont be shooting beyond 600. I did think this was ok but just means the remainder of my supersonic flight path isn't trued?

2. Captured environment in the shade and without interference from my hands or anything so that should be good.

3. I probably didn't fire enough rounds to get a solid grouping as such but each round fired did impact with the same elevation for the dial applied each time. The difference between first shot and the final trued dope was about 2 MOA hence the big speed difference.

4. Labradar MV was sitting at 2700 - 2720 fps. ES and SD were good throughout most of my load testing but to be fair when I got the final tweak of my loads seating depth I only ran a couple past the labradar as I already had a lot of data in that department so don't have the exact numbers for this exact load.

5. Unsure on barrel count as the rifle is second-hand unfortunately. Its a Tikka T3 though with 1:8 twist.


It shot good and accurate, its the speed that's got me slightly annoyed as I worked bloody hard to develop a load getting the performance I wanted only for the true to throw that out the window haha classic.

Do you you have the drop from 2-3 known distance?

Originally Posted by stagstalker

@JT_NZ

1. The one thing I definitely didn't do by the book was the range. I shot it at 657 yards rather then the 10% back from transonic which is about 1020 yards for this rifle. The reason I did this is because its a hunting rifle I wont be shooting beyond 600. I did think this was ok but just means the remainder of my supersonic flight path isn't trued?

2. Captured environment in the shade and without interference from my hands or anything so that should be good.

3. I probably didn't fire enough rounds to get a solid grouping as such but each round fired did impact with the same elevation for the dial applied each time. The difference between first shot and the final trued dope was about 2 MOA hence the big speed difference.

4. Labradar MV was sitting at 2700 - 2720 fps. ES and SD were good throughout most of my load testing but to be fair when I got the final tweak of my loads seating depth I only ran a couple past the labradar as I already had a lot of data in that department so don't have the exact numbers for this exact load.

5. Unsure on barrel count as the rifle is second-hand unfortunately. Its a Tikka T3 though with 1:8 twist.


It shot good and accurate, its the speed that's got me slightly annoyed as I worked bloody hard to develop a load getting the performance I wanted only for the true to throw that out the window haha classic.

Let us know if you find the issue. Other thing is scope height above bore? I've had to tweak this before. Easier on round actions as can measure diameter of action divide in half add height of rings then add to half tube diameter to get pretty precise height.

@stagstalker

It could be a number of things. Apologies for the following repeats but I will just cover off the troubleshooting checklist I was taught:

1. Incorrect range (measured or inputed)
2. Incorrect density altitude,
3. Incorrect spotting / poor grouping / not enough rounds to accurately determine mean point of impact
4. Inclination angle not a 0 degrees in "target" setting
5. Incorrect zero (including zero range wrong in Kestrel) (also make sure your units of measurement are correct for everything - I bounce between imperial and metric for different things)
6. Wind not set to zero in Kestrel
7. AJ not factored in
8. Direction of flight not inputed (this is more pertinent for CAL DSF)
9. Poor initial BC (not really a big concern as the CAL MV and CAL DSF process fixes this)
10. Incorrect bore height

Could be a couple other curly factors like barrel degrading BC more than normal or faulty scope but the issue is most likely one of the ten factors above. I recommend going through your "TGT", "Wind", "Gun" and "Enviro" menus and ensure that none of the ten points above are affecting your CAL MV. If no faults are found, I would redo your CAL MV, use the 0.293 G7 and do it out at Mach 1.2 which I am guessing will around 900m for you - pending DA. Flick me a call if you are still getting an abnormal result. I'll PM you my phone number.

Cheers,

JT

Are you measuring drop at distance without changing your dials from your zero?
If not, it could be your clicks are not exactly quarter MOA.
I have fired shots over two chronys (Magnetospeed and Prochrono) and had speeds only 1fps difference which gives me confidence in their speed measurements. I have also had scopes that are 0.28 MOA clicks.
Whatever the reason for the difference it doesn’t really matter in the big picture - the software gives you the correct dialling for your load from your rifle with your scope, which is the whole point!

@JT_NZ and @Cigar

What Cigar has just said rung some bells. Although I have not physically measured drop at distance or done a full on tracking test with this scope (Swarovski Z5), the previous rifle it was on had to adjust MV down when truing aswell (Although not to this extent but that was before I had the kestrel). I have just adjusted the gun profile setup to change from TMOA to SMOA and then re check the numbers and trueing data and this has brought my MV back up by 50fps to 2625. A number I would more readily accept as a variation from my labradar results. I would suspect some of this remaining error in MV is still a result of the difference in my scopes actual click adjustments but I wont know without doing a proper test on the scope. This never crossed my mind as I just expected sound results from such a quality scope but it all makes perfect sense! Trueing never lies and I think we have just resolved it. Thank you gents.

The swaro clicks are metric? x amount at 100m, (7mm/click? I’m out of town so can’t check mine, that would be 28mm per 4 clicks at 100m) so changing it to yards/Moa and that they may not be dead on could be the reason, The Duleys have mentioned this in some of there articles that some scopes aren’t correct MOA and there is a way of adjusting it in app.