# Help with drop chart

#### TBuckus

##### Well-Known Member
How or where do you make corrections to make a drop chart line up to actual data?
3 different drop charts are showing more drop than actual.
Here's my example
300yds
2.75MOA=1.75 high

500yds
6.75MOA= 3.75" high

600yds
9.25MOA= 6.375 high

I switched from G1 to G7 and the numbers change minimally.

With a pen or keyboard, write/type in the correct data on your drop chart. However, I think you are trying to ask something else.

Let's clarify some terms:

1) A Drop Chart is a document/electronic image with drops or ups for various distances given a zero range for a particular bullet launced at a particular velocity. The drops/ups are generated by a trajectory function in a Ballistic Calculator.

2) A Balistic Calculator is a program that estimates drops/ups for various distances given a zero range for a particular bullet launced at a particular velocity with various imputs for other variables; e.g., altitude and weather.

I think you are saying the values created by your chosen Ballistic Calculator don't match what you are actual seeing when you went to validate those calculations in the field.

Additionally, for anybody to help you; we need more accurate descriptions of your numbers - in your first example what are you trying to say: The calculator stated you would be 2.75 MOA high @ 300 but your actual result was 1.75" high? or you are zeroed at X distance and you dialed to 2.75 MOA to be dead-on at 300 but instead the bullets impacted 1.75" high? or are you trying to say something else?

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I have to agree there are some questions about your finding so the easiest thing to do is double check your inputs to your ballistics app or computer. Usually discrepancies are do to inputs on those apps. So I'd check these variables: 100 yard zero, scope height, elevation, actual velocity and so on.

…Also, how are you measuring your velocity that you're putting into the ballistic app? Which app are you using?

Garbage in, garbage out.

If any data is off, your shooting solution will be off. And each one that is off compounds the bad results.
Velocity (checked with a GOOD chrono)
Bullet BC
Shooting conditions like actual DA, temps, humidity, wind
Actual correct zero
Zero condiitions (same as shooting conditions)
Powder temp stability/MV Variation per °
Look angle

What ballistic app are you using? There are definitely some that are better than others. Shooter, Advanced Ballistics, and JBM Ballistics have all giving me correct and consistent drops. Streloc and Streloc Pro have never matched up with the above apps. I have found that with proper inputs, it makes a huge difference in shooting solutions. Both my rangefinders (BR2 and BR4) will give the same shooting solution as Shooter and JBM Ballistics if the inputs are correct, and Streloc will be off starting as close as 300 with the exact same inputs.

With a pen or keyboard, write/type in the correct data on your drop chart. However, I think you are trying to ask something else.

Let's clarify some terms:

1) A Drop Chart is a document/electronic image with drops or ups for various distances given a zero range for a particular bullet launced at a particular velocity. The drops/ups are generated by a trajectory function in a Ballistic Calculator.

2) A Balistic Calculator is a program that estimates drops/ups for various distances given a zero range for a particular bullet launced at a particular velocity with various imputs for other variables; e.g., altitude and weather.

I think you are saying the values created by your chosen Ballistic Calculator don't match what you are actual seeing when you went to validate those calculations in the field.

Additionally, for anybody to help you; we need more accurate descriptions of your numbers - in your first example what are you trying to say: The calculator stated you would be 2.75 MOA high @ 300 but your actual result was 1.75" high? or you are zeroed at X distance and you dialed to 2.75 MOA to be dead-on at 300 but instead the bullets impacted 1.75" high? or are you trying to say something else?
Correct. The calculator stated 2.75 MOA adjustment to be dead on, but my actual group was 1.75" high, etc.
I use jbm calculations for my data. The other resources I use to double check, and are pretty close are ballistic ae app, Bergers website.
My reloads were chrono'd with a Labradar. Zero conditions were different (59 vs 71, humidity a little less). I didnt bring the chrono when I went with friend to his range.
BC is per Berger website
Using R26, I'm not sure about MV per degree change
Shooting angle flat to maybe a minute uphill.
I believe I filled in all the data correctly, but I'll double check.

The other possibility causing an error is the scope isn't changing as much per click as it says.

Or the actual distance to target is different than what you're thinking.

Ok - thanks. Tell us what bullet, what velocity and what was your original zero distance and what elevation are you shooting at. Also, what scope?

A couple of things to also consider:

1) Could your rifle have been "bumped" and your original zero distance is no longer valid

2) Have you ever tested your scope for tracking; i.e, are the 1/4 inch MOA clicks really a 1/4 MOA. Consider, that 2.75 MOA is 11 clicks. and 1/4 moa at 300 yards is .785 inches; therefore, 1.75 inches is 2.23 clicks. Spread across 11 clicks, that means if each click were off by .05 MOA you would get 1.75 inches. Intersting enougth the calcualation was .048 for 500 and .027 for 600.

ShtrRdy and I were typing at the same time.

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Ok - thanks. Tell us what bullet, what velocity and what was your original zero distance and what elevation are you shooting at. Also, what scope?

A couple of things to also consider:

1) Could your rifle have been "bumped" and your original zero distance is no longer valid

2) Have you ever tested your scope for tracking; i.e, are the 1/4 inch MOA clicks really a 1/4 MOA. Consider, that 2.75 MOA is 11 clicks. and 1/4 moa at 300 yards is .785 inches; therefore, 1.75 inches is 2.23 clicks. Spread across 11 clicks, that means if each click were off by .05 MOA you would get 1.75 inches. Intersting enougth the calcualation was .048 for 500 and .027 for 600.

ShtrRdy and I were typing at the same time.
Garbage in, garbage out.

If any data is off, your shooting solution will be off. And each one that is off compounds the bad results.
Velocity (checked with a GOOD chrono)
Bullet BC
Shooting conditions like actual DA, temps, humidity, wind
Actual correct zero
Zero condiitions (same as shooting conditions)
Powder temp stability/MV Variation per °
Look angle

What ballistic app are you using? There are definitely some that are better than others. Shooter, Advanced Ballistics, and JBM Ballistics have all giving me correct and consistent drops. Streloc and Streloc Pro have never matched up with the above apps. I have found that with proper inputs, it makes a huge difference in shooting solutions. Both my rangefinders (BR2 and BR4) will give the same shooting solution as Shooter and JBM Ballistics if the inputs are correct, and Streloc will be off starting as close as 300 with the exact same inputs.
I will start of by saying thanks to those willing to help out.

OK, I did find a discrepancy in my data.
sight height was off. 1.5 vs 2.054 actual.
I made changes to the calcs but didn't correct everything.
Below is my chart.
 Trajectory Input Data Ballistic Coefficient: 0.307 G7 Caliber: 0.264 in Bullet Weight: 140.0 gr Muzzle Velocity: 3170.0 ft/s Distance to Chronograph: 0.0 ft Sight Height: 2.05 in Sight Offset: 0.00 in Zero Height: 0.00 in Zero Offset: 0.00 in Windage: 0.000 MOA Elevation: 0.000 MOA Line Of Sight Angle: 0.0 deg Cant Angle: 0.0 deg Wind Speed: 10.0 mph Wind Angle: 90.0 deg Target Speed: 10.0 mph Target Angle: 90.0 deg Target Height: 12.0 in Temperature: 71.0 °F Pressure: 29.92 in Hg Humidity: 40 % Altitude: 409.0 ft Vital Zone Radius: 5.0 in Std. Atmosphere at Altitude: No Pressure is Corrected: Yes Zero at Max. Point Blank Range: No Target Relative Drops: Yes Mark Sound Barrier Crossing: No Include Extra Rows: No Column 1 Units: 1.00 in Column 2 Units: 1.00 MOA Round Output to Whole Numbers: No Output Data Elevation: 3.669 MOA Windage: 0.000 MOA Atmospheric Density: 0.07335 lb/ft³ Speed of Sound: 1129.3 ft/s Maximum PBR: 399 yd Maximum PBR Zero: 339 yd Range of Maximum Height: 189 yd Energy at Maximum PBR: 2057.3 ft•lbs Sectional Density: 0.287 lb/in²

(yd)(in)(MOA)(in)(MOA)(ft/s)(none)(ft•lbs)(s)(in)(MOA)
Calculated Table
0-2.1***0.0***3170.02.8073123.30.0000.0***
25-1.2-4.60.00.13130.92.7723046.70.0244.216.0
50-0.6-1.10.10.23092.02.7382971.50.0488.416.1
75-0.2-0.20.20.33053.32.7042897.60.07212.716.2
100-0.0-0.00.40.43014.82.6702825.00.09717.116.3
125-0.1-0.10.70.52976.62.6362753.80.12221.516.4
150-0.4-0.21.00.62938.52.6022683.80.14725.916.5
175-0.9-0.51.30.72900.72.5692615.20.17330.516.6
200-1.8-0.81.70.82863.12.5352547.80.19935.116.7
225-2.9-1.22.20.92825.82.5022481.80.22639.716.8
250-4.2-1.62.81.12788.72.4692417.10.25244.417.0
275-5.9-2.03.41.22751.82.4372353.60.27949.217.1
300-7.8-2.54.01.32715.22.4042291.40.30754.017.2
325-10.0-2.94.71.42678.82.3722230.40.33558.917.3
350-12.5-3.45.51.52642.72.3402170.70.36363.817.4
375-15.4-3.96.41.62606.92.3082112.30.39168.917.5
400-18.5-4.47.31.82571.32.2772055.00.42074.017.7
425-22.0-4.98.41.92536.02.2461999.00.45079.117.8
450-25.8-5.59.42.02501.02.2151944.10.47984.417.9
475-30.0-6.010.62.12466.22.1841890.50.51089.718.0
500-34.5-6.611.82.32431.82.1531838.00.54095.118.2
525-39.4-7.213.12.42397.62.1231786.60.571100.618.3
550-44.7-7.814.52.52363.62.0931736.40.603106.118.4
575-50.3-8.416.02.62330.02.0631687.30.635111.718.6
600-56.4-9.017.52.82296.62.0341639.30.667117.418.7
625-62.9-9.619.12.92263.52.0041592.50.700123.218.8
650-69.8-10.320.83.12230.71.9751546.70.733129.119.0
675-77.1-10.922.63.22198.31.9471501.90.767135.119.1
700-84.9-11.624.53.32166.01.9181458.20.802141.119.2
725-93.1-12.326.53.52134.11.8901415.60.837147.219.4
750-101.9-13.028.63.62102.51.8621373.90.872153.519.5
775-111.1-13.730.73.82071.11.8341333.10.908159.819.7
800-120.8-14.433.03.92039.91.8061293.40.944166.219.8
825-131.0-15.2

It is still not what my actual data revealed. But better. If someone see's a discrepancy, I'd appreciate it.
I'll double check chrono and zero to see if changes. I haven't checked tracking on this scope. I would hope I can trust a NF.

Regarding hard info.
6.5-06
Berger 140gr VLD hunting. I've used a G1 BC of .6, .612. And G7 of .307
3170 fps, 100yd zero at 320 elevation R26 powder.
Scope- Nightforce NX8 4-32 FFP MOA

Looking at what hou are showing for solution vs actual drops:
300 yds you are about .75MOA high
500 yds you are about .75MOA high
600 yds you are about 1MOA high

Are you sure of your exact 100 yard zero? Could it actually be a 200 yard zero? Which would be about .75MOA high at 100. Changing your zero to 200 lines up a lot better with your actual drops.

Looking at what hou are showing for solution vs actual drops:
300 yds you are about .75MOA high
500 yds you are about .75MOA high
600 yds you are about 1MOA high

Are you sure of your exact 100 yard zero? Could it actually be a 200 yard zero? Which would be about .75MOA high at 100. Changing your zero to 200 lines up a lot better with your actual drops.
View attachment 384564
It's always possible. But I'm pretty sure I zero's it at 100.
The only rifle I zero at 200 is my one with a ZEISS rapid Z reticle

Looking at the totality of the above, let's think about your objective and the appropriate next steps and the easiest way to get the problem and an answer:

1) On a calm day get to a range and using the best available rests and benchrest techniques, confirm your zero is actually 100 yards. (As an aside, 200 yards is more standard for rifles set up for long range and for this rifle you might consider 250)

2) After you confirm your zero, shoot a group @ 100 yards. Dial 12 clicks up (I picked 12 because its going to identify even a small tracking problem and staying at 100 will eliminate other potential errors), shoot another group and you should have 3" of impact change. If you don't, you have a veru strong indication of a tracking problem.

3) Set up a tracking target (large grid of 1 inch squares with heavy blacklines), secure the rifle in places, and actually watch how your crosshairs track as you make dial adjustments.

You're setting up your calculator with 10 mph target speed. Unless you're actually shooting a moving target, you should set that value to 0. The program is trying to incorporate how much drop is needed to hit the target at specified ranges based on flight time of the bullet. Simply put, a target at 300 yards but moving at 10 mph will actually be farther away by the time the bullet arrives. Think Pythagorean theorem and that'll help understand. The longer the distance, the longer the flight time which equals more drop calculated. Unless you know the wind speed and direction for the day you shoot id recommend setting that value to 0 as well, at least for validation purposes. Try that and see if your numbers get closer.

Your actual BC can change with twist rate and velocity. I had a long chat with a Hornaday engineer one time about their 180gr eld. In my rifle with a 9.5 twist at 2850 he estimated my bc would be in the high 6's. That estimate is very close to the actual performance I am getting with 180 vld's.

If I were to re-barrel with a 7.5-8 twist he estimated the high velocity band might get as much as .810 (I don't remember the G7 BC equivalent but I do use it.)

He speculated my 180 vld's would probably have a similar result as the profiles are similar.