Trued vs measured MV, which one to trust?

[seraphs]

Tested 2 different 6 Dasher loads with my Kestrel 5700 Elite yesterday. I measured both MVs with Labradar, input the data to my Kestrel and captured the environment/wind...

For my first load (DTAC 115gr), the bullet drop up to 1075yds matched nicely with the calculation. The second one loaded with Lapua Scenar 105gr, however, missed quite a lot from the calculated drop. My measured MV was ~2830fps which was supposed to drop ~10.5mils at the target. Instead I hit it at 9.5mils. I had another target at 800yds which was also missed with the calculated elevation. I then calibrated my MV at 800yd and it showed 2926fps instead - almost 100fps difference from my measured MV! Does it sound normal/okay to you guys?

 

[iceng]

Was your BC entered correctly and zero conditions ? You said it was on a kestrel 5700e, so I'd guess it's all aligned properly.

Can argue with real world results. Record your dope and move on.

 

[Olen_4504]

If you are shooting at 800+ you should be truing your BC to get the elevation to line up, not your muzzle velocity.

 

[chase723]

Doesn’t sound right. Did you confirm drops with that new velocity at 100, 300, 500, 800, 1075? How many rounds did you shoot to establish your average velocity? What’s your ES? Is your rifle profile correct? While sight height doesn’t make a huge difference it does make a difference the farther out you go. Also, does your zero actually have no offset? Most often it does. You should true your BC to your rifle if your velocities are the same with a Labradar/Magnetospeed. They are accurate within 1%, which is 28 fps for your rifle.

 

[Dthomas3523]

100fps difference usually means you have either something else (like BC) that needed tweaking as well. What you’ve basically done is use velocity to tweak either multiple issues or another single issue.

Typically this means your dope won’t line up at closer targets now.

 

[azred33]

Both your bullets are in the Custom Curve library, if you are using those then there is no BC to adjust. 100 fps is not an unusual difference. The Cal MV number is a computed fact for your rifle's performance at distance. You don't mention if after CAL MV your holds for the Sceners were more accurate. Also, was 800 yds the distance the Kestrel recommended to do cal mv at?

 

[Tritiumdials]

100fps difference usually means you have either something else (like BC) that needed tweaking as well. What you’ve basically done is use velocity to tweak either multiple issues or another single issue.

Typically this means your dope won’t line up at closer targets now.

Is this a good rule of thumb to tweak BC if truing MV begins to surpass 100 fps difference?

 

[AllenOne1]

Trust the MV, true the BC.

 

[Dthomas3523]

Is this a good rule of thumb to tweak BC if truing MV begins to surpass 100 fps difference?

Well, there’s a lot that could be going on.

For me, if I’m changing the mv more than about 2%, I start looking at other issues. It may not be the BC. It could be something else the user inputted incorrectly.

Not just with software, but shooting and all things in general, when something is wrong, many times it’s the sum of a few errors and not just one thing.

 

[JLP.Payette]

True muzzle velocity at 600 true BC at 800+ then go back and tweek muzzle velocity at 600, works for me.

 

[seraphs]

Thanks for the inputs guys! A bit more context. I measured 7 shots of my Scenar load while warming up and confirming my 100yd zero with Labradar: 2849, 2841, 2830, 2838, 2849, 2827, 2837 (Avg 2838.7, SD 8.5, ES 22). I used the AB custom curve for my test. But I just reentered the data to Kestrel with G7 BC and found very little difference at 1075yd anyway (10.6 from AB vs 10.5 from G7).

I didn't pay much attention to targets <600yd as I hit all of them with the calculated drops. I started missing the 800 one (a fairly small target) so I decided to true my MV. From my measured MV, Kestrel showed that my transonic zone starts at 958yd. So I followed the instruction and trued my MV at 800yd (958 * 0.85 = 814), which showed 2926fps. When I moved to 1075yd, the drop with the trued MV didn't match quite well again (9.8mil calculated vs 9.5mil actual).

From what I read 90fps seems too much to most of you. I hope it's not the Labradar that gave me wrong measurements - that would be really bad as I never doubted its results and I had a lot of them for different loads/calibers. But as I mentioned earlier, my DTAC load matched pretty well with the Kestrel results though (diffs by 0.1-0.2mil at 1075).

 

[lash]

As already mentioned, either reshoot the suggested ranges to to verify actual drops at the least 600. Or adjust your BC using the limited info now and then reshoot for verification. Good data collection is important for you to see what is happening.

There is no reason to believe that your Labrador is giving you bad numbers. In fact, that your curve fits using the 115s tells you that. FWIW, I’ve had to adjust Lapua BCs upwards before to get a good fit.

 

[iceng]

Rezero. Good point.

 

[_Windrider_]

Was the gun rezeroed with the new rounds?

Or was the first rounds' zero used and you just dialed up to the prescribed dope?

I've seen people cut corners is all I'm saying.

This is where I’d start. I wouldn’t rezero the scope though. I’d leave it zeroed for which ever one you will shoot the most and just not the offsets for any other round. After that I’d tweak the MV around 600 and the BC around 800-1000 to get it to line up. If using the CDC, I’d just do whatever is recommendEd by AB

 

[azred33]

Your transonic zone started at 958 yds, so your 1075 shot is beyond the distance where the mv calc process functions (not much, I agree, but it is). For transonic distance use the custom curve and if needed DSF.

 

[seraphs]

Was the gun rezeroed with the new rounds?

Or was the first rounds' zero used and you just dialed up to the prescribed dope?

I've seen people cut corners is all I'm saying.

Yes I did rezero. Though at 100, the two loads diff by less than 1 click anyway.

 

[DocUSMCRetired]

Give these a read:

Ballistic Calibration - Applied Ballistics

By Doc Beech Ballistic Calibration is not as simple as we would all like to believe. While the process itself seems simple, it is actually deceptively easy. Making for simple mistakes that can cost you in the long run. The purpose of this short...

appliedballisticsllc.com

Firing Solution Checklist - Applied Ballistics

By Doc Beech The first thing that a lot of shooters consider when the predicted firing solution is not the same as the real world firing solution is “should I adjust the BC or the MV”. This depends on where the firing solution is off...

appliedballisticsllc.com

 

[beetroot]

If you haven't confirmed the data to be correct at closer ranges then you are just pissing in the wind.
It's no good truing the data to be correct at 800 yards if it's then out at every other distance.

If the chrony'd muzzle velocity was accurate with DTACs it'd be safe bet that it's correct for the Scenars also.

 

[JustSendit]

100fps calibration is outside my threshold for competition shooting when truing a MV. If I calMV at +/-10% in transonic and get more than 2 tenths of error I revisit that list Doc posted. I would closely ensure that list is checked, and everything in the kestrel is singing true. Definitely the zero is a major factor as plenty of others mentioned.

I don’t care if my calmv and chrono strictly agree. Currently my 6GT calmv at 2955 and chrono’d at 2930... I cleaned the long range stages at the last match I shot and no misses in general were attributed to my dope. If it works it works. But like I said there’s a threshold and 100 is pretty far off.