Dave King or JBM, Help! I found a ballistics problem!

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by goodgrouper, Feb 5, 2005.

1. goodgrouperWell-Known Member

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I currently own 5 ballistic programs and just realized that they all have one huge problem! They don't adjust for elevations changes correctly when it comes to the distances in which you zero your guns. Here's the scenario:

You are shooting at 3000 ft elevation. You sight in your rifle so that it is exactly 3" high at 100 yards. Your program says that for your particular load, your zero is 300 yards.

Now, you go up to 8000 feet on a mule deer hunt. You bring your laptop with you and punch in your pet load and enter in the elevation of 8000 feet. The program now says that you are shooting 3.2" high at 100 yards because of the elevation change. You assume that because the thinner air makes your gun shoot .2" higher than before that you are now zeroed at a little over 300 yards. But you look to the 300 yard drop chart and IT STAYED THE SAME!! You are still zeroed at 300 yards according to the computer, but you and I know that that isn't really the case in real life. You should now be zeroed farther than 300 yards.
Am I right??
How does the computer compensate for this phenomenon or does it? Is there a program that will actually re-adjust your zero at different elevations?
Very stumped. I might need six programs if there is one that does adjust.

My list of programs includes Infinity, Exbal, Precision shooters workbench, Excel, and Load.

P.S. Exbal will correct for this same phenomenon when it comes to angle shooting but not elevation changes as mentioned. For instance, the 3" high gun at 100 yards is zeroed at 300. Now shoot at 35 degree angles, and it shows that the gun is 3.3" high at 100 and it is now zeroed at 345. They got half the problem fixed. Why didn't they fix the other half?
I've been dazed and confused for so long it's not true.... /ubbthreads/images/graemlins/confused.gif Oh, sorry. Started singin' again.

2. Dave KingWell-Known Member

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GG

I assume the shooter will rezero at altitude so the 300 yard zero is a "local" 300 yard zero. I guess it could be done without assuming a re-zero but I'd need to think on it for a bit.

3. sscoyoteWell-Known Member

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Hey GG-- Exbal does recalculate the entire trajectory curve for changes in elevation.

4. goodgrouperWell-Known Member

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sscoyote,
How did you get it to do it or is it automatic? I enter in the scenario and my Exbal still keeps my zero at 300 yards instead of moving it out. Is it just slicing a fraction off a minute of each yard to the target?

Dave,
There has got to be a way to get the programs to calculate the correct trajectory without re-zeroing in the field to a local zero. It would be unwanted extra noise on a big game hunt to have to shoot paper at the new elevation.
Let me know what you figure out.
Thanks.

5. JBMWell-Known Member

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The issue here is that the elevation angle stays the same. When you go to the higher altitude, the same elevation angle will produce the effect you're looking for. On my online stuff, you would have to calculate your standard trajectory, note the elevation angle in the printout, then calculate another trajectory at the higher altitude and with this elevation angle entered and the option "Elevation Correction for Zero Range" turned off so that the program doesn't find a new elevation angle.

I agree, many programs do not allow you to do this -- that's why my online stuff allows the entry of an elevation (and azimuth angle) and has the option to turn the corrects off (same for MPM).

6. goodgrouperWell-Known Member

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JBM,
Can you elaborate on what exactly you are calling the elevation angle?
I am a little confused on the terminology. On my programs, elevation angle is the angle to target (like 10 or 35 degrees) and in my scenario, I am shooting on a theoretical flat rifle range at both elevations (3000 and 8000 ft.) so there should not be any angle.
Thanks.

7. 4ked HornWriters Guild

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[ QUOTE ]
You are shooting at 3000 ft elevation. You sight in your rifle so that it is exactly 3" high at 100 yards. Your program says that for your particular load, your zero is 300 yards.

Now, you go up to 8000 feet on a mule deer hunt. You bring your laptop with you and punch in your pet load and enter in the elevation of 8000 feet. The program now says that you are shooting 3.2" high at 100 yards because of the elevation change. You assume that because the thinner air makes your gun shoot .2" higher than before that you are now zeroed at a little over 300 yards.

[/ QUOTE ]

Air density is inversely proportional to ballistic coefficient. As the air gets thinner the effective BC rises. In other words if you shot a bullet with a 3000 fps muzzle velocity at sea level and then shoot this same bullet at 8000 feet elevation you should see similar results to shooting a higher BC bullet at 3000 fps MV at sea level.

What I'm trying to illustrate is that the thinner air should NOT Cause your bullet to have a higher trajectory at 100 yards. It should reach it's " 3" high" at a distance like 125 yards and it should be zeroed at something like 342 yards.(those numbers are just picked to illustrate my point I did not calculate them anywhere)

Gravity stays the same. The only difference should be a "stretching out" of the parabolic curve due to higher retained velocities at higher elevations.

For you to have a 300 yard zero at 8000 feet your bullets should print LOWER than 3" at 100 yards.

8. JBMWell-Known Member

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The elevation angle between the line of sight (what you see as you look through the sights/scope) and the barrel center line. This is the angle that you adjust when zeroing. Of course, this angle stays the same as you go to a higher elevation -- that's why you have to be able to input it.

9. JBMWell-Known Member

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[ QUOTE ]
Gravity stays the same. The only difference should be a "stretching out" of the parabolic curve due to higher retained velocities at higher elevations.

[/ QUOTE ]

The problem with this thinking is that there is a vertical component of the velocity that is retarded by gravity AND drag so with less drag it may go slightly higher -- less than an inch...

10. BrentWell-Known Member

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I don't have my Exbal PDA program right in front of me but, there's a check box somewhere in there that says "use field data". If this box is checked it will assume you've rezeroed at your current field conditions, and this would be closer to being zeroed at 300 yards than if it were using your original sight in conditions at lower altitude with the box unchecked.

I'll have to play with it a bit and see how mine's working.

I know you should see a remarkable change throught the trajectory curve if zeroed at sea level, use those sight in conditions then jump to 10,000' ASL.

POI may well be .5 MOA higher, so 2.25-2.50 MOA at 300 yards would now be more like it for a 300 yard zero.

The program should know what the total POI deviation from LOS would now be at whatever range. It knows the zero range, the atmospheric conditions at sight in and now in the field conditions and should modify the curve and resultant MOA correction at any range.

I wonder if the MOA correction Exbal gives on the calculate page matches the MOA correction in the drop chart page? Been a while now since I played with the program. Got to go locate my PDA come to think of it....

11. MikecrWell-Known Member

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I believe your right Brent-as stated. This is the reason I bought Exbal over all others.
My range is at SL in NC. I hunt in PA mountains, and do not re-zero. I don't have access to a range up there.
It's funny to me how many so called 'snipers' completely miss this. And how many 'sniper' marketed ballistic programs missed it as well.
Alot of assumptions I guess.

12. goodgrouperWell-Known Member

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Cool. This is all good. Let me know what you find.

13. 4ked HornWriters Guild

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[ QUOTE ]
The problem with this thinking is that there is a vertical component of the velocity that is retarded by gravity AND drag so with less drag it may go slightly higher

[/ QUOTE ]

I stand corrected. Upon further consideration I agree that there is an upward inertia that is affected by the density of the air as the bullet lifts above the line of sight. A resistance from the atmosphere above the bullet. Thinner air means that gravity is increasingly responsible for pulling the bullet back down.

I still maintain my first writing is correct when the bullet is fired from a bore that is parallel to the line of sight (not that this would ever happen in the real world.) That is, a trajectory not intended to rise tward the line of sight.

I also maintain that the distance at which the bullet would now drop through the line of sight (the zero distance) would increase by yards and agree with you that the increase in rise would be teency weency.