Not grasping the basics of uphill/downhill ballistics

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chutestrate

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You want to aim lower for a downhill AND uphill shot? I struggle with geometry and this makes no sense to me.

If I use a rangefinder and see a whitetail 100 yds out downhill my instinct is to aim slightly higher.
 
You aim the same in either case.

Because the actual distance uphill or down hill at extreme angles is shorter then it would be on level ground.

So there is less time for gravity to pull the bullet down from it's normal trajectory on flat land.

In actual fact, with a normal hunting rifle, at normal 100- 250 yard hunting range?

It's not worth worrying about much.

Unless you over-think it, and try to hold over or under to make up for the hill or tree stand you are on.

rc
 
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Think about how far HORIZONTALLY the bullet is travelling. Total distance doesn't matter as much.

For the most part, few shots any of us take are at such a steep angle that this matters at all.
 
Draw a 90 degree triangle.

If it looks like an L and you are in a stand, the bottom part is the distance gravity acts on the bullet. Now imagine you're at the bottom
Of the L and shooting up, same thing. The bullet path may be gravitationally shorter requiring less hold over.

an extreme example would be shooting straight down along the y axis at an animal, you would need no hold over or under since you are in the same gravitational plane.

The rate of drop is based in the horizontal axis. Also you have to imagine the plane the bullet travels through the animal.

Now in actual ballistics, the difference doesn't matter a hill of beans when up close, it's only when shooting long distance. Actual practice is much more important that theoretical.

You can buy one of those angle compensating range finders.
 
Dang. I try to give an image of why and you guys already beat me to the nuts and bolts of it. And here I was feeling smart. JeeZ
 
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Think about it, if you shoot straight up. Is there going to be any drop from bore axis? Nope, gravitational acceleration will force the bullet back down, but not in relation to the bore axis. If shooting 45 degrees inclined, Newton's gravitational acceleration constant g is still 32 FPS squared, but only half of that is exerting acceleration down in relation to the bore axis, the other half is slowing the bullet as it climbs up. SO, in affect, as it relates to drop from bore axis, gravitational acceleration is 16 fps squared.
 
IOW, aim lower on an uphill or downhill slope. :D There will be less drop. There are tables that will relate these figures to you in your caliber. You can probably google one, never tried. I live in the flat lands. But, the same thing rules the bow hunter shooting from a tree stand.
 
Don't worry about it unless you are shooting from an airplane or another extreme angle. It is so minute that shooter error will make up for it. Plus, you will have an excuse if you miss!
 
rcmodel said:
Because the actual distance uphill or down hill at extreme angles is shorter then it would be on level ground.

So there is less time for gravity to pull the bullet down from it's normal trajectory on flat land.
Click to expand...

If you range a deer at 300 yards and an angle of 0° and then you range another deer at 300 yards but at an angle of 30°, the flight time of the bullet to either deer is the same. Gravity acts on either bullet for the same amount of time. The difference is that the vector component of gravity perpendicular to the flight path of the bullet (making it drop from the point of aim) is less for the 30° shot than for the 0° shot, therefore the first bullet drops less over the same flight time.
 
an extreme example would be shooting straight down along the y axis at an animal, you would need no hold over or under since you are in the same gravitational plane.
Click to expand...

Your bullet would actually travel slightly to the east... not enough to matter, though.
 
WEG,

I can vouch for the bowhunters. I have spent much of my life in tree with a bow and can attest to shooting high when shooting at a sharp angle. Probably 80% of my misses have been to high.
 
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Oh boy, this is going to be harder than I thought. Even looking at the illustrations is confusing. If I range my target at a certain distance right now I think I want to plan on that distance, but compensate for the angle.

Shooting downhill I'll aim up to compensate, shooting uphill I'll aim lower.
 
No, up or down hill, you'd aim low either way to compensate because the vector at which gravity is affecting the bullet is the same.

You know, you can buy angle compensating laser range finders. Lase the target and it automatically tells you the range adjusted for angle of the shot. If I still hunted in the mountains, I'd get me one of these. They make 'em specifically for bow hunters, too. Once the range finder gives you the range, you just hold where you would for that range, no guessing for angle of the shot.
 
MCGunner, I just don't follow the logic. I'm looking at diagrams and it's greek to me. If I aim downhill my bullet returns to ground sooner so I compensate with a slight holdover. Uphill, the bullet will travel over my target unless I hold under the target.

Now I know I'm wrong, but I look at the diagrams I have, and what I'm drawing trying to understand what people are explaining and....nothin.

Guess I'll just buy one those range finders when I can afford one.
 
chutestrate,

Here's the easiest way to think about the situation.

When you're shooting at a target that is perfectly level with you, your scope is aimed level but the bore of the gun is aimed slightly upwards. That slight upward angle is to "loft" the bullet up enough to compensate for gravity's constant downward pull. If the bore and scope were both aimed level, the bullet would begin falling as soon as it left the bore, making it impossible to hit anything unless it is lower than the barrel.

So there's a slight upward angle to the bore compared to the scope. This is critical to understanding what's going on.

Now imagine what happens when you shoot STRAIGHT upwards as if you're trying to hit a bullseye 50 yards directly above you. Your scope is pointing straight up, but remember that the bore is angled "upwards" with respect to the scope. So the bullet will hit "above" the aiming point in the scope. It will hit a little behind your position on the target above you, "above" the crosshairs. So if you really want the bullet to go STRAIGHT up, you can't aim your scope straight up, you'll have to aim it a little "below" where you want the bullet to hit to compensate for the slight angle of the bore with respect to the scope.

Now imagine what happens when you shoot STRAIGHT downwards as if you're trying to hit a bullseye 50 yards directly below you. Your scope is pointing straight down, but remember that the bore is angled "upwards" with respect to the scope. So the bullet will hit "above" the aiming point in the scope. It won't go straight down, it will impact a little in front of your position on the target below you, slightly "above" the crosshairs. So if you really want the bullet to go STRAIGHT down, you can't aim your scope straight down, you'll have to aim it a little "below" where you want the bullet to hit to compensate for the slight angle of the bore with respect to the scope.

In other words, whether you aim at a significant angle up or down, the effect is the same. The bullet will impact a little higher than you would otherwise expect it to so you need to aim a little lower than you would otherwise.

Those are obviously extreme cases since we almost never have occasion to shoot straight up or down, but the general principle applies even to more normal shots when the angles are signficantly upwards or downwards from the shooter.

If you want to think about this in terms of gravity, this is what is going on.

That upwards angle of the bore with respect to the scope imparts a little bit of loft to the bullet to compensate for gravity. When you aim at something that's higher than you are, now the bore is angled up even more (due to your aiming upwards) and the effect is that you are now lofting the bullet MORE than you need to. So the bullet will go higher than you expect and therefore you need to aim low to compensate.

When you aim at something that's lower than you are, now it's ok for the bullet to fall a little bit because the target is below the bore. That means that the upwards angle of the bore with respect to the scope is providing too much compensation--a little too much loft than it should. So the bullet will go higher than it needs to and therefore you need to aim low to compensate.
 
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chutestrate, there are two easy angles between 0° and 90° that we can estimate and correct for in the field without any fancy equipment. One is 30° (-10%) and the other is 60° (-50%). Say you range a deer at 300 yards uphill or downhill (rangefinder with no built-in angle indicator, reticle etc.) and you estimate that it's at an angle of 30°. Take 10% off that range which is 30 yards so now you shoot it as if it's 270 yards at an angle of 0°. If you estimate the angle is close to 60 degrees, take 50% off the ranged distance and shoot it as if it's 150 yards at an angle of 0°.
 
JohnSka..thank you. Light just went on. Never thought about the elevation differences using the scope. Always think of my self as directly behind the barrel.

1858, that helps. Do you use the same formula for closer ranges such as bowhunting ranges?

Now that I think about it I missed my first archery deer by......shooting too high.
 
And this question is just out of general curiosity. if I were directly behind my rifle barrel, barrel coming out of my eyeball actually.

Would I then be compensating as I originally thought?
 
You'd still have to aim low to hit both upward and downward shots. That's because the bore would still have to be angled upwards a little to get the bullet out to a target level with your eye.

If the barrel came straight out of your eye and was pointed directly at the target, the bullet could never hit where you aimed/looked when you're looking more or less level. It would always hit low because gravity would pull the bullet down from level during the time it took to get from the muzzle to the target.

The straight up/straight down cases are useful because it's easier to see what's happening.

When you shoot level, the upward angle of the bore is counteracted by gravity and the bullet arcs back down to the target due to gravity's pull.

When you shoot straight up, gravity is no longer working to pull the bullet back on target because gravity is now working in a different direction with respect to the path of the bullet. So now the bullet keeps going along it's "upward" angle with respect to the line of sight and hits "high".

Here's a drawing that may help. Notice how the pull of gravity affects the bullets in the three cases. Basically it has very little effect on the trajectory of the up and down shots but it bends the trajectory of the level shot significantly.
attachment.php


The upward angle of the bore with respect to the sight line is to compensate for the effect of gravity on a level shot. But when you shoot at a significant upward or downward angle, gravity has less effect on the path of the bullet and therefore you have to keep in mind that the bullet will go higher than you expect it to.
 

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