up hill & down hill bullet drift

[crittergetter]

I can never remember for sure which way a bullet will drift when shot up hill VS down hill when shot at a target beyond a certain degree. I remember it being the opposite in that when shooting down hill the bullet can tend to drift up. How steep of an angle is it when this will become a factor? I don't know if this is even a factor when shooting a 270 with 130 grain under 300yds. Can someone please educate me? Thanks

[Val]

The issue in uphill or downhill targets is bullet drop. This is particularly an issue on very steep uphill or downhill slopes. Bullet drop is a function of gravity pulling the bullet down. The longer the horizontal distance the more the bullet drops. It is the horizontal component or distance that the bullet travels that determines how much the bullet will drop. A target at a very steep uphill or downhill slope may be at, lets say,three hundred yards but, because of the slope of the hill, the horizontal component/distance to the target can be significantly less than 300 yards. As a result of this, hunters will tend to shoot too high compensating for bullet drop that isn't really there because the horizontal distance is much less, because of the steep slope. The farther the target is and the steeper the slope the more a shooter will tend to miss or hit the target on the high side.

There are optical devices available that help calculate the true horizontal distance to the target but, who has time to play with one of those in a hunting scenario. Some people say to shoot for the opposite side of the animal such as the lung area. The opposite side lung target area for example,would be either higher or lower depending if the animal was up or down hill, than the side you were looking at. This would lower your shot thus hopefully compensate for the slope affect.

[crittergetter]

I never considered the horizonal gravity affect being the cause. I suppose the steeper the hill the less gravity drop so it would vary at every degree of slope. Thanks again Val. Very educational.

[NoCAL]

Easiest way I've found to understand is to draw a picture. Draw a triangle formed by putting yourself at the top (so like at the top of a mountain). Then draw one corner by putting a deer down below at some angle out from the first position. Now connect the two points by drawing a line straight down from the top of the mountain and a right angle across to the deer. If you were to use a laser range finder from the top to the deer, the distance might read out at 400 yrds. But as you can see from your drawing the actual horizontal distance between you and the deer is much less. If you remember geometry, A2 + B2 = C2 (can't make superscript but the 2's mean "squared") so the actual distance between you and the deer is C2/A2. Since you don't really know those numbers in the field what I do is if the range is over 300 yds and/or the angle is greater than 45 degrees, I compensate by not aiming as high. If you would normally aim at the backbone for a 300 yard shot, on a steeply angled shot aim for the mid-point of the body. It's really very subtle until you get out quite a ways or you have a very steep angle.

NoCAL

[larrysogla]

NoCAL is so right on!!! Uphil or downhill, the effect on bullet trajectory is the same.
The steeper the downhill or uphill angle, the higher the bullet will strike from the point of aim. You have to aim lower on an uphill or downhill shot. Why??? The shorter horizontal distance is only part of the answer. To relate in the simplest manner to what is happening, picture this extreme scenario in a rifle zeroed at 200 yds. If you aimed your rifle straight up, the line of aim is pointed straight up and the bore sight line is moving away from the line of aim in a straight trajectory instead of the balloon trajectory as in a normal horizontal shot. Therefore, the path of the bullet at 200 yds. is not "dropping" down to meet the line aim, it will continually move away from the line of aim & never cross that line of aim. The rifle is no longer zeroed at 200 yds. on a vertical shot, it is in fact not zeroed at all at any distance. That is what is happening on an upward or downward shot. You are gradually flattening the balloon trajectory of the bullet(within normal hunting distance of course) & thus the bullet will strike higher & higher as the angle of elevation or depression goes steeper & steeper. On an uphill or downhill shot, you have to aim lower to compensate for this flattening of the balloon trajectory of the bullet. The physics of it I understand but quite too complex for my words to explain. 'Nuff said.
God Bless. larrysogla.

[Speckmisser]

As NorCal said, it's about the horizontal distance of the bullet. The steeper the angle (up or down doesn't matter), the shorter the horizontal distance to the target.

For rifle hunters, it's only a true factor at extreme ranges or VERY steep angles (such as treestands or steep canyons).

[larrysogla]

I am really not entirely comfortable with the concept that the point of impact on an elevated or depressed shot is determined by the horizontal distance. We need some Physics Major to make that scientific explanation. I have a gut feeling that it is the vectoring of forces(as I remember from my High School physics days) that is involved. Meaning to say, a rifle zeroed at 200 yds. is fired horizontally at a target 100 yds. away. Then the same rifle is angled upward or downward sufficiently to produce a 100 yd. horizontal distance to a target above or below the shooter. I just have a gut feeling that the point of impact will not be the same as the leveled rifle and the angled(upward or downward) rifle at the same 100 yd. horizontal distance. I am not Physics savvy so this is just my HH(humble hunch). 'Nuff said(meaning, heck just aim that rifle lower on a downhill or uphill shot).
larrysogla.

[NoCAL]

larry,
Pretty sure you are correct, especially about the line of sight and mounting distance of scope over the bore. I've also heard your example of shooting straight up or down. Speckmisser provided a very good diagram illustrating my point about the horizontal distance being what is effected by gravity not the true over land distance to the target. Bottom line is like you said, don't worry about it until that angle gets very steep and the distance gets very long.

Good discussion.

NoCAL

[wmidbrook]

QUOTE

We need some Physics Major to make that scientific explanation. I have a gut feeling that it is the vectoring of forces(as I remember from my High School physics days) that is involved[/b]

I'm not a physics major but I aced physics in college. Basically you are right that there are differences between uphill & downhill shots. Namely, gravity when shooting uphill tends to slow the bullet faster and you'll end up with slightly more drop than you would on the same angled shot downhill. There's also slightly thinner air when shooting uphill but it's also negligable + not a guarantee due to atmospheric gradients, thermals, etc.

But, these differences are very, very minor and inconsequential compared to the horizontal distance.

Also, Norcal. a squared + b squared = c squared only for right triangles (Pythagorean's theorem) where c is the hypoteneus of a triangle (in this case, the actual slope the deer's standing on or the long side of the right triangle).

[NoCAL]

wmidbrook,
Check out the diagram above posted by Speckmisser. It is a right triangle. A is the distance from your point through the mountain to a point level with the deer. B is the distance from the end of point a to the deer and C is the actual distance from you to the deer. Therefore if you divide C2/A2 and solve for B2, you get the actual horizontal distance from you to the deer and the distance that gravity has to act on your bullet.

[wmidbrook]

I agree with you that it's a right angle. Not trying to step on anyone's toes. It's great info to know. Suprising how many people believe you have to hold over for uphill shots and shoot under for downhill shots.

I just thought I'd add the info so no one would get confused about a2 + b2 = c2 working for any triangle....works for any right triagle....it's just the engineer in me.

[Speckmisser]

QUOTE

Namely, gravity when shooting uphill tends to slow the bullet faster and you'll end up with slightly more drop than you would on the same angled shot downhill.[/b]

Sorry Bill, but you get an F on this one. Gravity affects the bullet over the plane ... not up or down hill. Objects are pulled toward the center of the sphere, so hills don't really affect it. Your explanation would suggest that gravity is actually stronger in a hill than on level ground.

Bottom line:

The distance affected on either shot (regardless of the severity of the angle) is based only on the bullet's horizontal flight. If you shoot up or down a 300 yard hill, but the horizontal distance is 150 yards, the bullet trajectory is exactly the same.

[Backcountry]

Originally posted by Speckmisser@Oct 12 2004, 09:46 PM
Sorry Bill, but you get an F on this one.###### Gravity affects the bullet over the plane ... not up or down hill.###### Objects are pulled toward the center of the sphere, so hills don't really affect it.###### Your explanation would suggest that gravity is actually stronger in a hill than on level ground.

Hey, any geophysicists out there? I'm not a card carrying geophysicist, but I am a geological engineer, and I've had a couple classes in geophysics. Kaster, check my “work” here…

Speckmisser, I've found one of those rare tidbits where you're not entirely correct.

From our perspective at the Earth's surface our planet is more-or-less spherical, and the resultant gravitational forces vector more-or-less towards the center of the Earth, more-or-less tangent to the Earth's surface. If the Earth was a perfect homogeneous sphere and did not rotate gravity would be the same everywhere on Earth, but this is not the case.

Let’s assign “g” to be the gravitational acceleration at the earth’s surface… for most engineering purposes, it’s sufficient to let g = 9.8 m/s^2, but geophysicists are interested in minute changes in “g”, down to the 0.00001 percent level, and thus they use different units for g, “Gals” (in honor of Galileo), and 1 Gal is 1 cm/s^2, and the common unit tossed about is the milligal (i.e., 0.001 Gal). Still with me or are we all asleep yet? Class… Buehler, Buehler… anybody, anybody…?

When measuring very tiny changes in the Earth's gravitational field, as geophysicists are wont to do, mountains, canyons, caves, ore bodies, dead bodies (gravity surveys are a proven forensic method), your elevation, and anything else that alters the density of the Earth's crust near where the gravitational measurement is being taken must be accounted and corrected for (these corrections are called the Bouger, terrain, and/or free-air corrections). Moreover, simple changes in latitude have a measurable effect on gravity. This is because the Earth is an oblate spheroid (think “squished sphere”) and not a perfect sphere, thus there is more mass and distance between the equator and the center of the Earth than between the poles and the center of the Earth. Also taken into account is how the rotation of the Earth effects gravity… the rotation of the Earth produces an outward vectored centrifugal force acting in the opposite direction of gravity. This effect is of course greatest at the equator and diminishes towards the poles. The net result (just like scoring up a nice muley rack and accounting for deductions) is that the Earth’s gravitational field changes from 978 Gal at the equator to 983.2 Gal at the poles (with a quick glance you engineers will confirm that both values are very close to 9.8 m/s^2).

Now, I will say from the point of view of rifle ballistics, the effects I'm speaking of is not measurable, not at all. I mention it solely as a sniglet of minutiae that you can file away for a later time when we play ultra-trivial pursuit around the campfire. But if you had a REALLY BIG GUN, and shoot shells from the equator to the poles, then you might want to take these factors into account (any rocket scientists amoung us?).

Yes, being a nerd has occasionally helped me score (and I don’t mean with deer).

Bottom line Speckmisser, the application of your answer is still 100% correct, but the "hills don't effect gravity bit" couldn't be ignored (by me). Gravity is actually stronger "in a hill".

Disclaimer… I did have to quickly consult my old geophysics textbook that I keep handy at my bedside to make sure I didn’t make any errors in this post.

Backcountry

p.s. So, who wants to be on my ultra-trivial pursuit team?

(Buehler…. Buehler…)

***Edited to add***

I forgot about the semi-diurnal tidal effects, which is actually something I don't have to look up. The solid Earth, more-or-less just like the oceans, exhibits what is known as "Earth tides". The Earth changes shape, and in some places the crust rises, and in other places it drops, with a period of about 12 hours. Thus gravity is going to change with time too... fortunately the change from mean is only about +/- 0.15 mGal which means it's also negligable from a ballistics standpoint. Could you imagine having to carry around Earth tide tables for long-range shooting? I have enough trouble figuring out the oceanic tides when I want to go abalone diving!

Tidbit about Earth tides

[Speckmisser]

OK... so I'm not even gonna try that one.

[wmidbrook]

QUOTE

Sorry Bill, but you get an F on this one.[/b]

Speckmisser, you may retake the test after doing some more homework...

Yes, thank you Backcountry.

I agree, the affects are negligable unless you are given a physics test and expected to calculate the point of impact down to the thousandths or ten thousandths like in a vacuum with no atmosphere.

From a hunting perspective, we're all on the right track.

[larrysogla]

May I add one more tidbit to this discussion, again. Horizontal distance?? Please allow me to bring back that extreme scenario. A rifle is zeroed at 100 yds. with a scope 2" above the boreline. The rifle is leveled horizontally, at O yd. horizontal distance with the target paper touching the muzzle, the bullet impact will be exactly 2" LOW(below the line of sight). Now go up on some high radio tower & at the 100 yd. elevation, point that rifle vertically downward & fire a shot with the horizontal distance covered as O yd. The bullet impact will most assuredly be greater than 2" HIGH(above the line of sight). The horizontal distance is the same(O yd.), but the bullet impact is 2" low on the horizontally leveled rifle & over 2" high on the vertically downward shooting rifle. We did not change the horizontal distance, we just changed the angle in which gravity acts on the axis of the bullet. The lesson!!! it is the vectored(angle of gravity on the axis of the bullet) gravity force that determines how high or low the bullet impact will be(within reasonable hunting ranges) . Hope this did not put to sleep some hunters out there. 'Nuff said.
God Bless. larrysogla.

[larrysogla]

May I clarify that at the radio tower, the target paper is 100 yds. vertically downward on the ground.

[jephs422]

Bullet drop does not change very much when shooting uphill/downhill. But the rifle will appear to shoot high. In fact it shoots high by almost the same amount whether you are shooting up or down. Therefore you must adjust your hold or change your scope when taking shots at high angle, especially as range increases.
WHen shooting at angles you
need the map distance not straight line distance. In otherwords, Imagine you the shooter
being in a high rise building and the target is sitting on a park bench at street level. You
use a laser range finder from your pos and determin that he is 220 yards away at a angle of
35 degrees slant. By using a calculator (Line of sight distance x cosine of firing angle) or
using a mildot master (from mildot enterprises) you determine that the correct distance to
put on the scope is 180 yds.

[Val]

Regardless of the hair splitters and the scientific input. In the real world, the horizontal distance to the target and the affect of gravity, as the bullet traverses that distance, determines the bullet drop. All the other factors are insignificant. The bottom line is that a shooter tends to shoot too high, when shooting at a target on a steep incline at a distance.

[larrysogla]

One last post on this topic & I am thru. For sure!! A rifle is zeroed at 100 yds. with the scope 2" above the bore. The Mid Range Trajectory is 1"plus(bullet impact is 1"plus above the boreline at 50 yds.). Keep the horizontal distance from the foot of the radio tower to the paper target at a constant 100 yds. Fired at a level position the bullet will impact at the exact line of sight(the rifle is zeroed at 100 yds.) at 100 yds. horizontal distance. Now start going up the radio tower & fire at the paper target(at a constant 100 yds. horizontal distance from the foot of the radio tower to the paper target) at the 100 yd. elevation, 300 yd. elevation & 500 yd. elevation & the bullet impact will progressively hit higher & higher from the line of sight. Meaning as the angle of descent becomes steeper & steeper the bullet impact starts moving higher & higher from the original 100 yd. zero EVEN THOUGH the 100 yd. horizontal distance is MAINTAINED from the foot of the tower to the paper target. It is NOT the horizontal distance but the angle in which gravity acts upon the flight path(axis) of the bullet that determines the rate of bullet drop from the BORELINE. When it comes to bullet impact, it is the rate of BULLET DROP FROM THE BORELINE (not the rate of bullet drop from a bullet dropped vertically to the ground) that determines how high or low the bullet will impact from the line of sight. Now I really have to quit. For sure!! Good luck. larrysogla.