Riddle me this? Ballistic calc vs reality mismatch...

[Stgraves260]

I'm no newb, but this one has me stumped al little. Went out today to really fine tune my zero and confirm drops before I start hard core "field practice" for the upcoming deer season, and I got a pretty severe disagreement with my ballistic software (Hornady 4DOF, Ballistic, and JBM online calculator).

Details...

7mm Rem Mag, 175 ELD-X, 1:8 twist, 2900 fps (30 fps ES, 9 fps SD). Bipod and bags, near perfect conditions (other than intense wildfire smoke). 70° with 0-3 mph "thermals" type breeze moving nearly 360° in map direction over the course of fire. Environmental variables changed very little over the 1 hour course of fire (72 to 68° F, 27.56 - 27.58" pressure) , confirmed with Kestrel and iPhone barometer.

Here's the 200 yard zero, with 9 shots, fired in 3 shot strings, with full cooling between strings (homemade barrel cooler).

View attachment 214123

Group is a hair over 1 MOA, which I'm just fine with (and is consistent with dozens of other measured groups). I pulled a couple of these an 1/8 or so, but unlike some statistically challenged folks, I'm not one to quote "If I do my part" or "Lucky they all missed in the same spot" 3 shot groups as the precision my rifle will achieve. This is my hunting rifle, and though I've recorded a few 1/3 MOA, 3-shot groups with it, and several 5 shot 3/4 MOA groups, that's not what we really shoot together. For ethical hunting, I want 9/10 confidence, or even 19/20. That takes big groups, and lots of them.

I digress...

Here's the 503 yd group. Same protocol, with 9 shots, 3 shot strings, fully cooled between strings.
View attachment 214130

This group is slightly better, though statistically speaking, barely so. Still perfectly happy with it. The offset to the right is most likely spin drift according to 4DOF.

This is the weird part...

Hornady 4DOF, Ballistic, and JBM online all suggest 6.7 - 6.9 MOA come up from a 200 yd zero (level) to 503 yds with a 2° incline. Taking into account the approximately 1/8 MOA offset in my 200 yd zero, that would equate to pretty much 6.5 - 6.75 MOA.

I dialed 6.75 MOA for this group, but it was 0.78 MOA high.

I've already noticed this high bias before, between 400 and 620 yds (can't go further till it rains a good bit, as there's too much fire danger to be mucking about in the other wheat stubble field). This isn't a one off "weird thing". It's relatively consistent.

So...Here's what I know...

1) Every shot went past the LabRadar. 2900 fps was average, and right in line with 50-75 other measurements of this load, and if anything, a hair on the slow side.

2) I did a box test at 100 yds a couple weeks ago with the scope (12 MOA right shoot 1 round, 20 up shoot, 12 left shoot, 20 down shoot, repeat 3X), and my scope (Sig Whiskey 5, 5-25x50) is within group size error of tracking perfectly.

3) I've used my LabRadar and JBM online to verify the BC of the 175 ELDX dozens of times. Values range from 0.680 - 0.720 G1, with an average of ~0.705 (Hornady quotes 0.689 -0.678 G1). With a carefully aimed LabRadar, I get reliable tracking out to 120-200 yds on 7mm projectiles, but usually right around 140 yds. I ran half of the 503 yd shots through the JBM calculator, and they averaged 0.695. Again, a hair on the low side.

4) I ran the BC up in my calculators until I got the right drop at 503 yds (~6.25 MOA), and it requires about a 0.85 BC.

5) I was shooting ESE, but according to Ballistic, Coriolis is only good for ~0.05 MOA at 503 yds.

So...Dubya Tee Eff?!?!

Hum. You got me. I don't even calculate for spin drift till after 700 yards, and that's only if I'm trying to shoot a 2" or 3" target. It's just that the calculations are so small you can't even adjust for them under 700 yards.

 

[Kingsgr8]

Not a drop of wind correction.

Wind was "dead calm" by most standards. I'm currently shooting in a fallow field (moon dust), so I can watch the consderable impact dust plume waft in the breeze. Tonight was lovely. Drifted away with the random nature of a drunken old man driving a walker with tennis balls on the feet.

Is it possible the moon dust affected the actual range of the range finder?

 

[Stgraves260]

Exactly. This is why I was saying to adjust the app so the dope from the app matches at 700 yards and everything else from 700 in would just fall into place.

Agree. I know apps differ so I would just pick the one that worked. Also a cosine indicator would come in handy for those longer shots.

 

[jstamper]

Have you tried adjusting the axial form factor?

^This was my first thought. In the 4DOF, app, you slide the form factor to account for changes the individual barrel makes on the bullet during obturation/engraving, supposedly.

 

[erikblank]

I know, stupid question, but did you take the coriolis effect into account? I didn't think the Hornady app did this, but I really don't use it.

 

[vancewalker007]

I'm no newb, but this one has me stumped al little. Went out today to really fine tune my zero and confirm drops before I start hard core "field practice" for the upcoming deer season, and I got a pretty severe disagreement with my ballistic software (Hornady 4DOF, Ballistic, and JBM online calculator).

Details...

7mm Rem Mag, 175 ELD-X, 1:8 twist, 2900 fps (30 fps ES, 9 fps SD). Bipod and bags, near perfect conditions (other than intense wildfire smoke). 70° with 0-3 mph "thermals" type breeze moving nearly 360° in map direction over the course of fire. Environmental variables changed very little over the 1 hour course of fire (72 to 68° F, 27.56 - 27.58" pressure) , confirmed with Kestrel and iPhone barometer.

Here's the 200 yard zero, with 9 shots, fired in 3 shot strings, with full cooling between strings (homemade barrel cooler).

View attachment 214123

Group is a hair over 1 MOA, which I'm just fine with (and is consistent with dozens of other measured groups). I pulled a couple of these an 1/8 or so, but unlike some statistically challenged folks, I'm not one to quote "If I do my part" or "Lucky they all missed in the same spot" 3 shot groups as the precision my rifle will achieve. This is my hunting rifle, and though I've recorded a few 1/3 MOA, 3-shot groups with it, and several 5 shot 3/4 MOA groups, that's not what we really shoot together. For ethical hunting, I want 9/10 confidence, or even 19/20. That takes big groups, and lots of them.

I digress...

Here's the 503 yd group. Same protocol, with 9 shots, 3 shot strings, fully cooled between strings.
View attachment 214130

This group is slightly better, though statistically speaking, barely so. Still perfectly happy with it. The offset to the right is most likely spin drift according to 4DOF.

This is the weird part...

Hornady 4DOF, Ballistic, and JBM online all suggest 6.7 - 6.9 MOA come up from a 200 yd zero (level) to 503 yds with a 2° incline. Taking into account the approximately 1/8 MOA offset in my 200 yd zero, that would equate to pretty much 6.5 - 6.75 MOA.

I dialed 6.75 MOA for this group, but it was 0.78 MOA high.

I've already noticed this high bias before, between 400 and 620 yds (can't go further till it rains a good bit, as there's too much fire danger to be mucking about in the other wheat stubble field). This isn't a one off "weird thing". It's relatively consistent.

So...Here's what I know...

1) Every shot went past the LabRadar. 2900 fps was average, and right in line with 50-75 other measurements of this load, and if anything, a hair on the slow side.

2) I did a box test at 100 yds a couple weeks ago with the scope (12 MOA right shoot 1 round, 20 up shoot, 12 left shoot, 20 down shoot, repeat 3X), and my scope (Sig Whiskey 5, 5-25x50) is within group size error of tracking perfectly.

3) I've used my LabRadar and JBM online to verify the BC of the 175 ELDX dozens of times. Values range from 0.680 - 0.720 G1, with an average of ~0.705 (Hornady quotes 0.689 -0.678 G1). With a carefully aimed LabRadar, I get reliable tracking out to 120-200 yds on 7mm projectiles, but usually right around 140 yds. I ran half of the 503 yd shots through the JBM calculator, and they averaged 0.695. Again, a hair on the low side.

4) I ran the BC up in my calculators until I got the right drop at 503 yds (~6.25 MOA), and it requires about a 0.85 BC.

5) I was shooting ESE, but according to Ballistic, Coriolis is only good for ~0.05 MOA at 503 yds.

So...Dubya Tee Eff?!?!

There are a lot of good suggestions in the posts so far. With everything working normal it's impossible to predict where your bullet BC is gonna be at longer ranges from a given rifle. Frankly if you intend to possibly shoot farther at game I wouldn't change anything until you can collect data at longer ranges to compare to 500. If 500 is your limit then I'd adjust the BC or velocity to compensate in what ever your range finding solution is for the elevation error and leave it. I go through this every time I spin up a new load. I end up shooting 300-1000ish to fine tune BCs/velocities to put my G7 range finder spot on.

 

[Mc Fraser]

I'm no newb, but this one has me stumped al little. Went out today to really fine tune my zero and confirm drops before I start hard core "field practice" for the upcoming deer season, and I got a pretty severe disagreement with my ballistic software (Hornady 4DOF, Ballistic, and JBM online calculator).

Details...

7mm Rem Mag, 175 ELD-X, 1:8 twist, 2900 fps (30 fps ES, 9 fps SD). Bipod and bags, near perfect conditions (other than intense wildfire smoke). 70° with 0-3 mph "thermals" type breeze moving nearly 360° in map direction over the course of fire. Environmental variables changed very little over the 1 hour course of fire (72 to 68° F, 27.56 - 27.58" pressure) , confirmed with Kestrel and iPhone barometer.

Here's the 200 yard zero, with 9 shots, fired in 3 shot strings, with full cooling between strings (homemade barrel cooler).

View attachment 214123

Group is a hair over 1 MOA, which I'm just fine with (and is consistent with dozens of other measured groups). I pulled a couple of these an 1/8 or so, but unlike some statistically challenged folks, I'm not one to quote "If I do my part" or "Lucky they all missed in the same spot" 3 shot groups as the precision my rifle will achieve. This is my hunting rifle, and though I've recorded a few 1/3 MOA, 3-shot groups with it, and several 5 shot 3/4 MOA groups, that's not what we really shoot together. For ethical hunting, I want 9/10 confidence, or even 19/20. That takes big groups, and lots of them.

I digress...

Here's the 503 yd group. Same protocol, with 9 shots, 3 shot strings, fully cooled between strings.
View attachment 214130

This group is slightly better, though statistically speaking, barely so. Still perfectly happy with it. The offset to the right is most likely spin drift according to 4DOF.

This is the weird part...

Hornady 4DOF, Ballistic, and JBM online all suggest 6.7 - 6.9 MOA come up from a 200 yd zero (level) to 503 yds with a 2° incline. Taking into account the approximately 1/8 MOA offset in my 200 yd zero, that would equate to pretty much 6.5 - 6.75 MOA.

I dialed 6.75 MOA for this group, but it was 0.78 MOA high.

I've already noticed this high bias before, between 400 and 620 yds (can't go further till it rains a good bit, as there's too much fire danger to be mucking about in the other wheat stubble field). This isn't a one off "weird thing". It's relatively consistent.

So...Here's what I know...

1) Every shot went past the LabRadar. 2900 fps was average, and right in line with 50-75 other measurements of this load, and if anything, a hair on the slow side.

2) I did a box test at 100 yds a couple weeks ago with the scope (12 MOA right shoot 1 round, 20 up shoot, 12 left shoot, 20 down shoot, repeat 3X), and my scope (Sig Whiskey 5, 5-25x50) is within group size error of tracking perfectly.

3) I've used my LabRadar and JBM online to verify the BC of the 175 ELDX dozens of times. Values range from 0.680 - 0.720 G1, with an average of ~0.705 (Hornady quotes 0.689 -0.678 G1). With a carefully aimed LabRadar, I get reliable tracking out to 120-200 yds on 7mm projectiles, but usually right around 140 yds. I ran half of the 503 yd shots through the JBM calculator, and they averaged 0.695. Again, a hair on the low side.

4) I ran the BC up in my calculators until I got the right drop at 503 yds (~6.25 MOA), and it requires about a 0.85 BC.

5) I was shooting ESE, but according to Ballistic, Coriolis is only good for ~0.05 MOA at 503 yds.

So...Dubya Tee Eff?!?!

I think it all boils down to static vs dynamic, you are using a static system to input into a dynamic one. Ballistic calculators assume everything static, wind speed, scope elevation/windage being true, etc. then you take a shot in a dynamic environment and some things change. On top of that there is human error. I learn to leave with it or try a few ballistic calculators.
I am curious of your homemade barrel cooler.

 

[Stgraves260]

I would not worry about spin drift at that short of distance. I would be more concerned about my cosine

 

[Stgraves260]

See if this helps.

 

[Stgraves260]

Just a 5 degree difference at 500 yards will make a world of difference. So instead of calculating for 500 yards you would need to calculate for 498 yards. That's A 6' difference in drop.

 

[OutsideSeth]

Were you using HCD (Horizontal Corrected Distance) from your range finder or true distance? You mentioned a 2-degree slope, so I am wondering if that was being accounted for twice. That would make your POI high, but I am not sure if it would account for that much without doing the math.

 

[Onesock]

Adjust your bullet speed in your app. Raise the speed a bit.

 

[azsugarbear]

One thing I have noticed over the years with different VLD designed bullets: they all don't fit exactly into the G7 model calculations. The G7 model (like the G1) is based roughly on a certain style of bullet. The subtle differences in design of a VLD bullet make iut such that some will conform to the model better than others. I had on rifle/bullet combo that would hit 1/4 to 1/2 moa higher at 400-700 yds, then was dead on from 700-1,000 yds, and then would drop 1/4 moa beyond 1,100 yds. I made no changes for the higher hits at closer ranges as they were still hitting in the 'dead' zone. The lower hits at 1,100 yds and beyond I compensated for by dropping the BC number for that specific velocity band.

Having said that, I have never experience as wide a variation in moa as your specific example. My advice would be to put the rifle away and not make any changes to scope/rifle/etc. Go out again when things have calmed down and re-shoot the same scenario and see if the results are duplicated. If they are, then you can tackle the issue with some of the solutions mentioned in this thread. Sometimes, it is a combinaton of many small things that only existed in the moment. Best of luck to you.

 

[Lionel Boyd Johnson]

I had the same sort of issue shooting a very accurate Rem 40X rifle off of a bipod. Switched to front and rear bags, and the trajectory more closely matched the calculated points. Of course, it could be your scope does not adjust in exact MOA as was already mentioned. -Ed