BC Correction

[Doc Holiday13]

I was able to shoot my 7PRC beyond 1K finally and have some questions about getting correct bullet drop.

175 ELDX with MV of 2899 fps @ 8680 ft elevation, 74 F and 12% humidity

Took some initial shots at 806 yards and all first round impacts. Using sig Kilo10k with weather station and AB gave a solution of 4.7 mils

Next target was 1242 yards. AB gave me the solution of 8.7 mils.. Came up short and ultimately needed .5 mils to get on target.

*edit info add: I also plugged all my data into hornady 4DOF and it gave me a correction of 8.9 . Still .3 mils short

I know there are variables in shooting this distance. This is a hunting rifle, not target, so I don't plan to shoot an animal that far, but it is bothersome that my ballistic solution was off. Sig has velocity calibration and 2 target drop calibration. a velocity calibration would obviously be the wrong approach. Would the the measurement difference between 2 targets be the solution that would get my ballistic solution corrected using the Sig BDX software?

 

[Ledzep]

My approach to this, regardless of the software (4DoF is ideal, and I don't just say that because I work at Hornady-- how it works is fundamentally different and better than BC based solvers) is centered around data collection.

First and foremost, you must be absolutely positive that your optic is zeroed, tracks, and if there are deviations you need to know what they are. Have enough rounds through it, run it out, bring it back to zero, to have a good baseline that you KNOW your foundation is solid.

Next, I write down the day's environmental conditions, and within as short a time as is reasonable such that the conditions don't significantly change (understanding hunting rifles, especially magnums, take time to cool off) I will shoot at 200-400yd, 500-700yd, and 900-1200yd. At least 10 rounds per yard line on a big and clean enough target that you can measure it (ruler or reticle). Write down what you dialed, write down what correction would have brought you to exactly waterline. ALSO IMPORTANT: Write down wind speed and direction and corrections.

With 3-4 yardlines worth of data in the same conditions (or at least with different conditions documented), it allows you to go back and adjust settings and see what works. It's very easy to correct for 800-1000yd then come back and find out that 0-400yd is all jacked up. Having it all layed out very quickly squashes confusing back and forth BS in this regard.

If your MV is solid, I would guess that you need to bump your Axial form factor up a percent or two and you'll see it move things maybe .05-.1 at 800yd and make up that .3 at 1242. Verify your scope-over-bore measurement, too.

BC solvers essentially scale a standard projectile's Cd vs. Mach values across the Mach regime and if you could lay your actual Cd vs. Mach over that scaled approximation you could see the gaps where the Cd is either higher and/or lower than the scaled standard. Anywhere the Cd is wrong, the drag will be calculated that much wrong, the solver then time step iterates and that error carries through the next step, then the error at that step is added and this process continues for the entire solution. As such, it's imperative to minimize those errors. Unfortunately, without seeing your specific projectile's Cd vs. Mach (doppler radar) it's very hard to know where you need to average the BC scalar to curve fit the closest. Regardless, there will almost certainly eventually be errors and the best way to do this is to created banded BC profiles. Essentially make a profile for 0-600yd, 600-900yd, 900-1100yd, 1100-1300yd etc. with a different BC used at each band.. You will have to play with things (record actual data and then fuck with the solver to see what works) to see how wide or narrow the bands need to be. It is tedious.

Unless the specific projectile's Cd vs. Mach curve lines up perfectly on top of the scaled standard projectile's, I think you will struggle to find a single BC value that will work from 0-1200+yd. YMMV.

 

[SIDS01]

I was able to shoot my 7PRC beyond 1K finally and have some questions about getting correct bullet drop.

175 ELDX with MV of 2899 fps @ 8680 ft elevation, 74 F and 12% humidity

Took some initial shots at 806 yards and all first round impacts. Using sig Kilo10k with weather station and AB gave a solution of 4.7 mils

Next target was 1242 yards. AB gave me the solution of 8.7 mils.. Came up short and ultimately needed .5 mils to get on target.

*edit info add: I also plugged all my data into hornady 4DOF and it gave me a correction of 8.9 . Still .3 mils short

I know there are variables in shooting this distance. This is a hunting rifle, not target, so I don't plan to shoot an animal that far, but it is bothersome that my ballistic solution was off. Sig has velocity calibration and 2 target drop calibration. a velocity calibration would obviously be the wrong approach. Would the the measurement difference between 2 targets be the solution that would get my ballistic solution corrected using the Sig BDX software?

Were all of the 1242 yard shots exactly 2899 fps? If they were not measured individually with a LabRadar or similar, that is one of the more likely problems. Change the velocity 20 fps and compare the impacts to the 806 and 1242 yard firing solutions. It'll be a lot larger at 1242. In the absence of actual velocities for those shots, truing with velocity is the preferred solution at 1200 yards.

Was 8680' something you entered or the DA calculated from the local pressure and temperature? This is one of the more common errors but if the range is that high, it's unlikely you'd even see the impact if you double dipped or canceled the air density correction.

The problem with adjusting the BC at 1242 yards is it's still a bit close and will require a fairly heavy handed approach that'll blow up longer solutions. The flip side is the barrel can affect the BC. With a fresh custom barrel, 4DOF often puts them over the top for me. Factory barrels generally need the BC estimate brought down.

It could also be a chain of inputs that stacked up in the same direction. 10-20 F difference between the air temperatures at the shooting position compared to the bullet flight, 20 fps on velocity, 10 yards on the range, 1200 yards if far enough the heading matters, 1 moa is ~0.3 mil......

 

[John Glidewell]

Our process is simple and has worked on many rigs. We run ZCOs so unless I see something really wrong we do not do tracking test. Have yet to see an issue with any of them. First we verify zero at 100. We make sure it is 100 yards too, not 101. We verify the data in the Kestrel. We shot a 611 yard plate with a 2" waterline through the middle. Most folks miss the or skip the water line, not good. If we have to make an adjustment on the scope to be in the water line we will use the true velocity function and get that to match. Then we move to a 966 target and do same thing. Normally it is good to go, but sometimes it is a tenth high or low. That should be the final velocity truing. So far I have been good to a mile. Only projectiles we had to mess with BC on was Hornady. Using the AB custom curves for Bergers or DTACS has been spot on. We would only adjust BC if we can shoot to transonic and need to make an adjustment. I think the thing folks miss in this process is the water line. Just hitting the target is not enough. If you are hitting 3" below your aim point, your calculator is off. This truing process has worked for us on everything we shoot.

 

[secondofangle2]

I would also confirm the range

The Sigg laser kind of sucks

If you’re on any sloping terrain at all, you don’t know if it is ranging your target or the ground 50 yards behind it

 

[John Glidewell]

I would also confirm the range

The Sigg laser kind of sucks

If you’re on any sloping terrain at all, you don’t know if it is ranging your target or the ground 50 yards behind it

Great point. Our firing line is set up so the line runs through the turret of our scopes, another piece some folks miss. (do not measure from the muzzle, you adjust/measure for the turret) We put up a rather large paper target at our firing line. We drive out to our targets and range back to the target on the firing line. The person at the firing line ranges us. We did this a few times with 5 different range finders we got to demo just to see how close they were. We personally use the Vortex 4k HD range finders and they have been really good for us. I tested mine by using a contractors measuring tape and measured 100 yards and it was spot on exactly reading 100, so I am fairly confident it is accurate. By doing this we ensure what and how we test/true is the same every time. A lot of folks take it for granted and say close enough. Folks forget that shooting at distanced with any error is multiplied by compounding factors the farther you go out. BS in = BS out! Edit to add, we also range with the range finder in a tripod, much easier.

 

[secondofangle2]

I don’t think whether you zero at 100 yards or 102 yards makes one iota of difference if you check the ballistics calculator.

However, if you were checking a scope for tracking, you need to be down to the inch in your distance to the tall target. I have several posts on this. Typical laser rangefinder are accurate to within one yard, which is not enough.

So you need to do like you say and use an actual measuring tape or I use a LEICA DISTO D2 precision rangefinder

 

[John Glidewell]

The point I failed to make with that was make sure your data is good and matches. We do 100 as it is set at our range. But if I have 102 for zero in calculator but actually shot my zero at 100, then the output data will not be accurate and get worse the farther I shoot. Our hunters typically do a 200 yard zero, so valid point. So long as the data points entered are correct. BS in = BS out.

 

[ggmanning]

I only true MV. Zero at 100 yds, shoot a truing bar or water line at the range the kestrel tells me is the best for my data entered, Adjust as needed. Has worked very well for several different rounds to a mile. I use the Litz/AB custom drag data models for everything I shoot vs G1 or G7

 

[secondofangle2]

I still am not following.

For my 7PRC - if I have 100 yard zero, it's 65.19 MOA to 2000 yards
If I have 103 yard zero, it's 65.17 MOA to 2000 yards.

But if you're off by just 3 yards at 2000 (you put in 2000 and it's really 2003 yards), now your come up is 65.37, almost a full click more - so you will be .2 MOA low, or 4 inches at 2000 yards. And that's just 3 yards ranging error at distance.

TO figure out what matters and what doesn't, you just have to plug test numbers into your calculator.

Now, if your zero POI is a tiny group centered .25 MOA above the POA at 100 yards, that needs to be accounted for, and most calculators have something like that "zero offset" because sometimes you're less than a click away from POA/POI alignment, and going a click in the right direction just makes the error worse in the opposite direction.

But I'm back to square 1 - if your zero target is 100 yards versus 103 or even 105, that don't make one iota of a difference at range. Unless you're doing a scope tracking test at 100 yards - then you need to be accurate to an inch or a fraction thereof else you will build in error to your "correction"

 

[John Glidewell]

Apples to Oranges. Just changing the numbers in the calculator really does nothing.

If you shot your zero at 103 yards thinking it was 100 and you put a 100 yard zero in your ballistics calculator - your equations at distance will never line up.

It is actual numbers and performance vs assumptions.

Just plugging in arbitrary numbers is the definition of BS in = BS out.