Help: Gun vs.Target - steep shots, barometric adjustments with big elevation change?

[pstimac]

Originally Posted by Buffalobob

it will be 1 inch low due to the altitude changing air density. and the result will be 0.75 inches low at 1000 yards.

The adjsutment factors I gave you in the first post were derived by running the ballistics repeatedly in JBM with changes in altitude while holding temperature constant and then running with constant altitude but different temperatures. They were developed for 1000 yards.

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I did this also and saw the difference up vs down but was wondering if the changing altitude pressure change (pressure differential) was actually being taken into account or if this difference was only a result of the different bullet trajectories from being shot up vs. down.

[Buffalobob]

Well, before you asked the question and got me to thinking about it, I had always assumed that the difference in the uphill drop and the downhill drop was caused by gravity accelerating the bullet when it is going down hill and slowing the bullet when it is going up hill. Thus causing a longer time in flight and more drop on the uphill shot than the down hill shot. Whether the altitude density change factor is included in JBM, I do not know because it is a smaller effect than the gravitational effect.

The PDA version of Exbal does not take into account whether you are shooting uphill or downhill and therefore does not automatically take into account altitude and temperature differences between the shooter and the target. You could enter the midpoint data if you wished and make a correction that way as a first approximation. Of course you realize you have the uncorrected larger error of up versus down from gravity but fortunately they are partially compensating errors.

[Augustus]

I noticed that my Atrag had no way of dealing with + or _ inclination angles. That really bothered me so I did a little reading. Most of the material was on the Horus Vision Website, according to this material gravity has the same effect on a projectile whether it is shot uphill or downhill. This is how it was explained. To simplify the explanation imagine standing on a hill, you see a nice sis/six bull and your Sworo. says he is 707 yds away. When you aim at him your inclination angle is 45 deg. If you think about it you have now described a right triangle with the hypotenuse being represented by the 707 yds that is your line of sight. The leg oppisite your position is the actual horizontal distance to the bull which is 500 yds. Gravity only acts on the projectile for the actual horozontal distance. It would make no difference if you and the bull changed position the come-up would be the same regardless if the bull was above or below you. The question the gentleman asked about shooting from one elevation to another can become rather complicated due to the interaction of BC,temp, and baro. press. Ther interaction will change when shooting uphill or downhill. Thinking about this makes my head hurt and there is no way the average Joe is going to sort all of this out while looking at the bull. You are either going to have a ballistic program to sort this out or avoid shooting through drastic elevation changes. I played with a few exterme scenerios and if all the forces line up to pull the same way at the same time the error can be significant.

[Buffalobob]

Things normally work like this unless you got Diet Smith and his anti-gravity machine available.

As you shoot uphill, the acceleration of gravity "G" is subtracted from the vertical component of the velocity "Vy" and slows the total velocity of the bullet.

As you shoot downhill, the acceleration of gravity "G" is added to the vertical component of the velocity "Vy" and increases the total velocity of the bullet.

At 1000 yards for the 7AM there is about a 30 fps difference in bullet velocity from shooting uphill versus downhill

Now then you throw into the mix that air is less dense in the uphill direction and the bullet is slowed less from air resistance and is the opposite when you shoot downhill. Thus, you find that the effects of shooting with and against gravity is reduced by the changes in density.


I recommend that you go and read the link in my first post.

[Augustus]

I did and now I have a headache. Also another thing occurred to me, if shooting downhill at a very steep angle and there is a substantial head wind,would this tend to cause the projectile to drop more than expected. It seems the projectile would be acted upon similar to a crosswind.

[pstimac]

....nice drawing only thing is is... the horns aren't big enough on that deer.



"Thus, you find that the effects of shooting with and against gravity is reduced by the changes in density."

?? wouldn't the effects be amplified....reduced force when shooting up hill because of g as well as increased pressure at lower altitude (more drag slowing the bullet initially)? and when shooting down? increased v because of g as well as less pressure (less drag)

At times like these I wish I would have taken physics....

[Augustus]

You stated there was 30 fps difference in shooting up and down hill with the 7AM. What angle?