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Range finder results at up and down angles.

 
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  #1  
Old 07-22-2010, 12:09 PM
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Location: Texas
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Range finder results at up and down angles.

When utilizing the on Line JBM ballistics calculator for one of my bullet/speed combinations (300 WSM - 175 gr JLK) the results show -223.7 at 1000 yds when you input -30 degrees for shooting downhill and -219.9 at 1000 yds when you input 30 degrees for shooting uphill or 3.8 inches difference. With another software the difference was 4.4 inches for the same load.

The ballistic software that is sold by Night force does not allow you to input a negative number for shooting downhill. This software only allows apositive number for degrees. Gerald Perry who wrote the NIghtforce software says that the point of impact is the same for uphill and downhill shooting.

Can anyone shed some light on this? Does shooting uphill vs shooting downhill result in a difference in the required drops?
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Old 07-22-2010, 08:37 PM
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Re: Range finder results at up and down angles.

Quote:
Originally Posted by Hunter66654 View Post
Does shooting uphill vs shooting downhill result in a difference in the required drops?
Incorrect information deleted.

Apologies all around!

Roy

Last edited by royinidaho; 07-24-2010 at 10:51 PM. Reason: I was incorrect!!!
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  #3  
Old 07-24-2010, 11:40 AM
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Re: Range finder results at up and down angles.

Sure, the cosines of + and - any angle are the same, but that's only part of the calculation. There are two "accelerations" on a bullet in flight. The acceleration of gravity is only 1 or 2 percent that of the acceleration (deceleration) from air drag, but it's always present. With any uphill shot a bullets velocity will be a little slower than with a downhill shot. The difference being from the gravitational forward component (forward with respect to the bullets spin axis) on the bullets travel over it's trajectory. As range increases and the angles are greater that difference becomes increasingly important. If the software ignores the sign of the angle it's at least a less complete model than one which includes the gravitational effect. For a detailed explanation see "Modern Exterior Ballistics" by Robert F. McCoy. He was the ballistics researcher at the Aberdeen Proving Grounds who wrote the original McTraj software which most modern ballistics programs are based on.

The JBM software handles the gravitational component of up/down shooting correctly. I'm not familiar with the Nightforce software, but if it has no provision to enter the sign of the angle to the target it cannot be correcting for both uphill and downhill gravitational effects..

All ballistics programs are incomplete models. Most software models don't include corrections for such things as spin drift, bullet yaw, precession, wind jump, Coriolis effect, etc but those are usually smaller than the measurement errors in major input parameters, particularly wind. If an under 4 inch calculation error for +/- 30 degree shots at 1000 yards concerns you then don't use the Nightforce software. On the other hand who can estimate downrange wind speed well enough when shooting at 1000 yards to get deflection errors under 4" even if the software is perfect?

Last edited by LouBoyd; 07-24-2010 at 11:57 AM.
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  #4  
Old 07-24-2010, 12:25 PM
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Re: Range finder results at up and down angles.

Thanks for the reply. You are right, 4" at 1000 yds is small and I am sure it is smaller than I could hold in a hunting situation. I am just trying to find out who's software is the most accurate and handles the up vs down angles correctly. The NightForce ballistics software is the Xbal program written by Gerald Perry with NightForce's name on it.
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Old 07-24-2010, 10:47 PM
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Re: Range finder results at up and down angles.

Lou,

I stand corrected!!!

Thanks for the post.

Roy
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  #6  
Old 07-25-2010, 09:49 AM
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Re: Range finder results at up and down angles.

In my opinion when evaluating exterior ballistic software a very important consideration is ease of use. If you have to type in information for 50 seconds what use is it for hunting.
At least the software should have memory which remembers the setup of you rifle. That includes scope over bore height, muzzle velocity for your loads, temperature coefficient (if known), and bullet BC for an appropriate G() model.
What else is needed?
What must be entered in the field during the shooting session? Enough info to determine air density which may be elevation, air temperature, barometric pressure, and humidity, or use an air density gage.
Then for each shot you need distance to the target (from a range finder), up/down angle to the target. Then there'd the sticky problem. How to measure wind. Pull out the Kestrel and read the direct and crosswind speed to 3 decimal places at the place you're shooting, enter the crosswind in the provided space,, set you're knobs as indicated and take the shot. It's that simple.

No it's not. For example,you're on a hill and your target is on another hill 652 yards (measured) away. It's a 14 degree down angle shot (measured) . The wind reads 5.3 mph perpendicular to your shot and it's from an angle 100 degrees from your target. But you're on the side of the hill and you're surrounded by trees.. What is the wind 100, 200, 300 and 400 yards from your location toward the target? You look though your scope and binoculars and see no mirage and no blowing plant particles. The trajectory is through clear air.

Does having a complete software ballistic model matter? Will gravitational up/down angle correction matter? How about bullet jump or spin drift effects. Those won't matter at all if the wind between 100 and 400 yards really is between 15 and 25 mph and you don't know it. So you punch in all the information 5.3 mph you have into the calculator, take the shot and miss, or worse, take a non lethal shot and the deer runs to cover before you can take another shot. You hike to the spot and spend the rest of the day trying to track a weak blood trail. Not an enjoyable way to spend a hunting trip.

Can you blame the software? Your rifle? Or your shooting skill? It can only be your fault for not realizing that wind is typically the least measurable parameter in shooting and often impossible to determine with reasonable accuracy as there may be no visible indicators. Even it you can guess or estimate the downrange wind velocity no commercial software I've seen allows you to enter downrange wind vectors for segments of the trajectory. The effect of downrange winds are likely to be much greater than up/down gravitational effects or spin drift. I don't have a solution other than to not place more faith in a ballistics computer than it deserves and choose what shots to take accordingly.

I'm not at all against the use of ballistic software, but none that I've seen put error bars on the effect of uncertainty on each of the input parameters. The book "Applied Ballistics for Long Range Shooting" by Bryan Litz (a frequent contributor to LRH) has a good discussion of how error sources combine. Even a perfect ballistic model can't do anything to correct for bad input data.

Last edited by LouBoyd; 07-25-2010 at 10:42 AM.
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  #7  
Old 07-25-2010, 10:28 AM
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Re: Range finder results at up and down angles.

And.....there's the accuracy of the range finder. As in 0.5% beyond 800 yards for example. (nearly 1/4 MOA).

Things add up quickly.
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