Elevation error builds as distance increases ...

[rustyinbend]

Here's the situation:

• Tikka T3x TAC A1 in 6.5 Creedmoor with Leupold Mark 5HD 5-25
• Hornady 6.5-CM American Gunner 140gr ammo
• Velocity meticulously documented via LabRadar
• Ballistics via Ketrel 5700 Elite with Applied Ballistics
• Sub-MOA tight groups at 100 yards to zero scope

Spent the day at a clinic shooting various targets from 300 to 1,600 yards at well-known distances. Results out to 700-ish were spot-on ... one shot - one hit. After that, the rounds seemed to hit consistently high. By the time I got to 1,600 yards, I was applying elevation for at least 50-yards less distance. It was "progressive", meaning that I had to aim a little low to hit at 800, and a lot low to hit at 1,600 ... misses on Kestrel ballistics were 1/4 mil high at 800 yards, and about 3/4 mil high at 1,600 yards.

That's the bad news. The good news is that I was able, with good spotting, to adjust quickly and hit at all long distances. Seeing the reaction target light up at 1,600 yards is pretty freakin' fun.

Is this normal and expected when pushing a 6.5 Creedmoor to unnaturally long distances?
Is there a way to predict this and build it into the ballistics for a better 1st-round ELD hit rate?
Would this phenomenon go away with better / heavier ammo ... like 147 ELDM's?

Thanks for sharing any advice or experiences that might help with 1st-shot-hits next time.

Side note ... I have longer-distance rifles that don't do this (300 Win Mag specifically) ... but I wanted to see just how far I could accurately shoot my 6.5-CM Tikka.

 

[Powder_Burns]

Did you true your mv and bc in your ballistic solver? Do you have aerodynamic jump, spin drift, and coriolis turned off in your kestrel?

 

[308pirate]

The bullet doesn't lie. Write the dope that you used down and forget about the kestrel.

 

[david8989]

Have you tall target tested your scope for any tracking errors prior to the class? Sometimes scopes will track fine for the first 5-7 mils then start to build error. Also I've found that when using ballistic solvers to get pinpoint accuracy out past 1000yds all those little variables that others have told you to turn off come into play. You combine those variables, a poor wind call, a small angle to the shot, with a slight truing or zeroing error (even a perceived perfect zero has error) and you've stacked enough variables that as you extend your range they start to show up. Check out the Hornady 4dof app since your using their bullet. They have steered away from using BC and use an actual doppler model of their bullets that I've found, when set up properly, tracks amazingly well out to the max I've shot it @ 1400yds

 

[Rocketmandb]

The bullet doesn't lie. Write the dope that you used down and forget about the kestrel.

[insert meme pointing up at the above post]

A pencil and water-resistant notebook are your friends.

 

[LawnMM]

Check your scopes tracking, are you using the custom drag curve or a g1/g7 value?

 

[phlegethon]

[insert meme pointing up at the above post]

A pencil and water-resistant notebook are your friends.

Better to do both. Writing stuff down is important but we have technology for a reason. If you true the Kestrel data you’ll be able to get better firing solutions when environmentals change or you go to different distances.

 

[Rocketmandb]

Better to do both. Writing stuff down is important but we have technology for a reason. If you true the Kestrel data you’ll be able to get better firing solutions when environmentals change or you go to different distances.

When I'm just out plinking steel, what I like to do is get DA, temp, etc. from the Kestrel first. Then I like to try to judge the wind. I used to be a windsurfer and spent countless hours on the shore waiting for the wind to hit just the right speed, so I'm pretty good at it. Then based on everything, I like to look at the closest DOPE I've got and take the first shot. If I don't have DOPE for the exact range and conditions, I'll use the calculator, but I'll guess first to see how close I can get.

Far more fun than looking at a gadget or a screen, and you tend to commit it to memory a little more effectively.

 

[Precision Underground]

The variables are scope tracking, BC of the bullet in the software vs reality, muzzle velocity in the software vs reality, and environmental factors like aerodynamic jump and visual oddities due to light refraction/mirage. If it were me I’d true MV and/or BC to make the software give me the correct solution, assuming it’s not a tracking issue.

when you say you were dead on at the shorter distances were you hitting exactly where you were aiming or were you hitting the steel? Being 10% low at 700 may still be an impact, not so much at 1600. You need to make sure you are on a waterline before you call the dope correct.

 

[rustyinbend]

Did you true your mv and bc in your ballistic solver? Do you have aerodynamic jump, spin drift, and coriolis turned off in your kestrel?

Yup ... velocity is accurate via LabRadar and BC is accurate based on the specs.
Also checked Spin Drift and Coriolis are double-checked on in the Kestrel.
All very good things to check.

 

[rustyinbend]

Check your scopes tracking, are you using the custom drag curve or a g1/g7 value?

G1 ... How does a custom drag curve work?

 

[rustyinbend]

Have you tall target tested your scope for any tracking errors prior to the class? Sometimes scopes will track fine for the first 5-7 mils then start to build error. Also I've found that when using ballistic solvers to get pinpoint accuracy out past 1000yds all those little variables that others have told you to turn off come into play. You combine those variables, a poor wind call, a small angle to the shot, with a slight truing or zeroing error (even a perceived perfect zero has error) and you've stacked enough variables that as you extend your range they start to show up. Check out the Hornady 4dof app since your using their bullet. They have steered away from using BC and use an actual doppler model of their bullets that I've found, when set up properly, tracks amazingly well out to the max I've shot it @ 1400yds

Good advice ... thanks!
Not likely there's a scope error on this Leupold Mark 5 HD ... I have several and shoot them on other rifles to very long distances with great accuracy.

 

[Dthomas3523]

Yup ... velocity is accurate via LabRadar and BC is accurate based on the specs.
Also checked Spin Drift and Coriolis are double-checked on in the Kestrel.
All very good things to check.

Based in “specs” meaning what the box says?

If so, then you haven’t and need to true up your BC.

 

[rustyinbend]

Based in “specs” meaning what the box says?

If so, then you haven’t and need to true up your BC.

I'm using the Kestrel bullet assignment. Can you point me in the general direction of where I can read up on how to do that ... true up my BC?

 

[LawnMM]

If you have an Elite Kestrel with Ballistics try using the custom drag curve instead of the g1 for that bullet and see if it's more accurate. RTFM.

 

[phlegethon]

G1 ... How does a custom drag curve work?

That is likely your problem or a big part of it. The G1 ballistic coefficient uses a drag model that is really designed for flat base roundnose bullets, and doesn’t match modern low drag boat tail rifle bullets well. Even if the G1 BC is “accurate” at some speed, use of the G1 model will tend to greatly underestimate drag at some parts of the bullet’s flight and overestimate it at others. The G7 model is far more accurate and should always be used preferentially.

Unless you have a custom drag curve. Even the G7 model is just that, an idealized model. It will still have some error in estimating drag for a particular bullet and speed. A custom drag curve actually measures the speed of the bullet in flight as it travels and slows, allowing for the best possible accuracy in prediction (limited only by environmental unknowns).

So if you are using Applied Ballistics software in some way, use a custom drag curve if they have one for your bullet. Otherwise use G7.

 

[rustyinbend]

If you have an Elite Kestrel with Ballistics try using the custom drag curve instead of the g1 for that bullet and see if it's more accurate. RTFM.

Based on this advice, I ... RTFM ... and am headed out in a few days to see if the custom drag curve gives me better results at long distances. Thanks.

 

[702lineman]

If you wrote down the dope you had from today then change kestrel to the custom drag model, put in chrono velocity and see if it lines up with what you actually shot. If it doesn't, then true up your kestel. Done. There are plenty of videos on YouTube on how to do this. The main thing is you already have recorded data from shooting today.

 

[702lineman]

My magnetospeed says my 107s are running 2930 with sd of 2. But to get the dope to line up the kestrel thinks it is running 2983 fps. Is it whacky? Yup, but as long as it gives me the right data out, I dont give a shit. And it does.

I used to use ballistic arc. Only had to true 1 rifle with it. I adjusted the mv till it lined up under 800 yrd, and then adjusted g7 bc till it lined up past 800. I had recoded dope using waterlines on targets to 1100.

 

[Gleedus]

Personally I would feel that G1 vs G7 isnt a big deal for supersonic flight predictions a lot of guys make it out to be but it is more app dependent if there is a difference.
I have found the CD from AB to generally to be good but not always.
If I have exact MV I prefer not to change MV in the app for trueing as it can cause error when changing to different temp.
It is very likely your predictions will have to be off by a 1/4 moa mid flight to true the 1600 but dont sweat it.
Most of my experience it with 22lr which is more picky with what MV you use because BC is MV dependent. So in reality when you change your MV you are also modifing your BC.

 

[phlegethon]

Personally I would feel that G1 vs G7 isnt a big deal for supersonic flight predictions

It's math. Your feelings don't matter. G7 is a much better model for modern long range bullets...

Most of my experience it with 22lr which is more picky with what MV you use because BC is MV dependent.

...and there you go. 22LR bullets aren't modern long range bullets. They are round nosed, flat based bullets that look exactly like the G1 model. Moreover, they are usually subsonic, which makes the drag far less and the drag model much less critical. (And by the way, BC varies a lot more with supersonic bullets.)

 

[Gleedus]

It's math. Your feelings don't matter. G7 is a much better model for modern long range bullets...


...and there you go. 22LR bullets aren't modern long range bullets. They are round nosed, flat based bullets that look exactly like the G1 model. Moreover, they are usually subsonic, which makes the drag far less and the drag model much less critical. (And by the way, BC varies a lot more with supersonic bullets.)

I said personally feel to avoid coming across to pointed to help encourage in the correct direction.
The facts are G7 typically is a better drag model for 22lr because we are dealing with subsonic flight and the drop curve is a different "shape" then a bullet in supersonic flight and i mean comparing the same bullet not different bullets. To say that this bullet or that bullet matches the G7 model better is a false security because our modern VLDs are NOT matches to G7 either. Our midrange boattails are closer to the G7 model shape.
A good ballistic calculator in supersonic flight will give correct results once trued both with G1 and G7 and both will need trued. Some apps will be more precise with G7 then G1 but this is very much app dependent. AB tends to be more precise with G7 and many people use AB and form opinions without learning the math behind or testing other apps.
Changing MV can create problems with VLD bullets as well i run a 6mmSLR and i shoot from -30 to 30C⁰ and have proved this in real life dope mostly when in the lower temp.
This shows up even more with the 22lr because it is more sensitive to velocity.
You may also get your drop data trued by changing MV but it will affect your wind drift calculations.

 

[nutation]

That is likely your problem or a big part of it. The G1 ballistic coefficient uses a drag model that is really designed for flat base roundnose bullets, and doesn’t match modern low drag boat tail rifle bullets well. Even if the G1 BC is “accurate” at some speed, use of the G1 model will tend to greatly underestimate drag at some parts of the bullet’s flight and overestimate it at others. The G7 model is far more accurate and should always be used preferentially.

Unless you have a custom drag curve. Even the G7 model is just that, an idealized model. It will still have some error in estimating drag for a particular bullet and speed. A custom drag curve actually measures the speed of the bullet in flight as it travels and slows, allowing for the best possible accuracy in prediction (limited only by environmental unknowns).

So if you are using Applied Ballistics software in some way, use a custom drag curve if they have one for your bullet. Otherwise use G7.

Bryan Litz has a wonderful explanation of what the ballistic coefficient, and the various drag models, actually mean in Chapter 2 of Applied Ballistics for Long-Range Shooting. Briefly, the ballistic coefficient given by bullet manufacturers is basically the sectional density of the bullet divided by a form factor. The form factor is determined by comparing the idealized drag model to the actual bullet drag. Some bullets are closer to the ideal drag model than others. The Sierra .308 210 grain MatchKing is a nearly exact ballistic match to the idealized G7 drag model; the form factor is 1.000, so that bullet's G7 ballistic coefficient is equal to its sectional density.

 

[rustyinbend]

Custom drag curve, trued BC, and reloading for single-digit SD ... all worked together to solve this. Kestrel numbers are spot-on for this rifle now out to almost a mile. Thanks guys ... for the great advice.

 

[Steel head]

The bullet doesn't lie. Write the dope that you used down and forget about the kestrel.

bullet does what the bullet does.


With a bit of tinkering you can easily get your kestrel to jive with actuals.


Make sure your come ups are consistent, double check to make sure you lock your air conditions after you get a reading.
Then use the MV adjustment and a BC adjustment and/or the subsonic function for the far stuff if needed.