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

entoptics

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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).

IMG_5023.JPG


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.
IMG_5021.JPG


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?!?!
 

entoptics

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Did you dial/hold for any wind on the 503yd shots?
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.
 

Hawkeye Pierce

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Do your calculators have a G7 setting?
I prefer the Applied Ballistics software, I use the shooter app as it fits my current financial availability lol.
That's still kinda odd though. I'm sure it's going to be one of those simple answers that we overlook.
 

J E Custom

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Rarely is the ballistic performance the same as advertised. The factory numbers are under optimum
conditions and only account for one BC. When establishing a drop chart it is prudent to have three over the distance desired and velocities at those points. The BC can change because of performance at different velocities and should be taken into account once you have good actual drop data. then you can find the actual Bullet BC and use it in your programs for best results.

Actual Bullet BC's are never the same as advertised where I hunt and shoot so I have to adjust them to match the drop chart to get good results. I also like to have a 5 to 10 mile wind at 90o to help the calculation better match the actual Bullet BC's.

J E CUSTOM
 

entoptics

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Error in velocity, error in zero, both or a input that shouldn’t be.

Did you laser 200y when you zero’d?
Ranges were taken with a Vortex Fury 5000, within half a yard from end of muzzle.

Top pic above shows the zero group (1/8 MOA high and left), fired right before 503 yd group, and all shots went past a LabRadar, so I don't see how velocity or zero can be the issue...

3 Different ballistics programs agreed on ~6.8 ± 0.15 MOA with following inputs...

0.7 G1 BC
27.56 inHg uncorrected pressure
70° F
40% humidity
1.8" Sight over bore

As mentioned above, I'd need 0.85 plus G1 (0.5 G7) to get the group down to the bullseye.

Playing around with pressure/temp, and my measurements would have had to be off the equivalent of 8000 ft elevation gain, and I think I would have noticed if it was 150° F...

I'm used to some variation from reality to software, particularly at >>500 yds, but 3/4 MOA at 500 is perplexing.

Seems like it has to be in the scope (despite box test results), so I'll revisit that when I get the chance.
 

dfanonymous

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Ranges were taken with a Vortex Fury 5000, within half a yard from end of muzzle.

Top pic above shows the zero group (1/8 MOA high and left), fired right before 503 yd group, and all shots went past a LabRadar, so I don't see how velocity or zero can be the issue...

3 Different ballistics programs agreed on ~6.8 ± 0.15 MOA with following inputs...

0.7 G1 BC
27.56 inHg uncorrected pressure
70° F
40% humidity
1.8" Sight over bore

As mentioned above, I'd need 0.85 plus G1 (0.5 G7) to get the group down to the bullseye.

Playing around with pressure/temp, and my measurements would have had to be off the equivalent of 8000 ft elevation gain, and I think I would have noticed if it was 150° F...

I'm used to some variation from reality to software, particularly at >>500 yds, but 3/4 MOA at 500 is perplexing.

Seems like it has to be in the scope (despite box test results), so I'll revisit that when I get the chance.
It’s not necessarily that your chrono is wrong but one or many of these are off as they are the ONLY things that will effect your dope to 500.

Scope tracking
range estimation errors
temp/pressure altitude error
BC error
and last but not least, velocity error.

The issue is for a 7mm rem mag you are still just outside of max pbr distance lol. If you’re using a box BC then it’s not going to be so off that it’s going to throw your solution off .8 moa @ 500.

It’s not the ballistic apps as I’m sure people use them just fine. None of your verticals drifts will have affect at such close range. Chances are that it’s one of the usual and basic things listed above. Variation in software is not truing it, or understanding it. I have no ballistic profile that is outside of reality. Some of these cartridges out to 2 miles as far as drop is concerned.
 
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Euler

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About 1/4 MOA of SD at 500 could be possible, but seems fairly large. Also a very minor breeze could account for a portion (or all of it) of it, and some cant could also account for a small portion. But I agree, not worth worrying about at this point.

As far as elevation discrepancy, I wouldn't touch BC at 500 yards.

Double/triple check all initial inputs in your solver (environmentals & rifle set-up). Confirm your LRF is accurate an not set to meters or something simple. Check your scope to ensure it is actually tracking correctly. I'd also say get yourself a 100 yard zero instead of 200 yards zero to further remove errors. From there, tweak your MV to get waterline hits out to 700 yards (give or take), confirm at 100 yard intervals, and tweak BC at the furthest distance you can shoot (if necessary).
 

Barrelnut

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I'm with JE. This happens to me all the time, even after careful tall target testing, and vertical tracking tests on the scope with calibration in the ballistic app.
I like to use Applied Ballistics custom bullet profiles and therefore cannot change the BC. So, what I do is shoot at 600~700 yards and figure out the drop needed to be spot on. Then I adjust the velocity of the bullet in the app to match those results. After that everything from 700 to 200 yards just falls into place. Out to 1K is good too...
 

zr600

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I would use the g7 bc numbers and also use the advanced part of the ap were it asks barrel twist, elevation, and bullet length too. I have had pretty good luck with the free vortex balictic ap.
 

Vol1975

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find your know dope at various range.
I personally do 100 500 700 and 900

what every u shoot...

the 500 yard will most of the time tell you how good your gun will shoot.
Once you get this established it’s just playing good around with the many parameters that are associated with most quailty apps to find a match- I have applied Ballastic and it’s a pain to get right but once I get certain parameters established it’s pretty good.

parameters being
stuff like
scope height, correct speed( remember most crongraph have a (1–2% fudge factor), Humidty, weather conditions, click height in scope that are correct adjustment (maybe a click is .35 inch and not .3), your zero is correct, etc. etc.
a little bit here and there make a lot of difference.
just have to play with it and repeat test.

Anyway. Not a set it and forget it thing
more of a pain in butt To get it right.
 

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