Not grasping the basics of uphill/downhill ballistics

[MCgunner]

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?

The answer is YES because the laws of Newtonian physics still apply. If you are shooting 45 degrees UP or DOWN, gravitational acceleration is working on that bullet, relative to the bore axis, at 45 degrees and will thus be half of what it was shooting perpendicular to gravity (flat to the horizon).

The formula concerning drop of a bullet or any other object, if I can write it on a keyboard, is D=1/2g T^2 (T squared) D is distance of drop, g is the gravitational acceleration constant on the earth (32 fps squared) and T is time or in ballistics it's time of flight. One can see that if you halve the gravitational acceleration to 16 fps squared by angling the shot up or down hill, you decrease the distance the object/bullet drops with time.

You're making me remember physics. That hurts my head.

And, Interlock, Newton was a BRIT! Come on, dude! ROFL

 

[.44Rich]

If your still having a hard time with all the math they sell range finders with built in angle finders. They usually get used in archery, but mine goes out to 550yards. Works pretty well. Its a nikon prostaff i think. You aim designate and then it tells you the yards you should aim for, does both true distance at line of sight, and your aim distance for angle comp.

 

[jmr40]

Using this illustration.

http://www.google.com/imgres?sa=X&r...ndsp=8&ved=1t:429,r:13,s:0,i:120&tx=117&ty=76

A range finder will read 30 yards to the deer. Because of the angle, the arrow only has to actually travel 21 yards to the deer. It would be the exact same shooting up hill. If the shooter were on the ground, and the deer were 21 yards away, but eleveted 20 yards higher the range finder would give a false reading of 30 yards.

 

[brainwake]

I missed my first crossbow shot because of this. Shot right over the top of it. I hunt in rugged land with lots of hollers. My first mistake was not really calculating the distance properly. What I thought was about 50-60 was really 40-50. Then I didn't compensate for the downhill angle. So I missed....lesson learned. Deer lives another day. So I went out and bought me the Leupold bow range finder. Now the first thing I do when I sit down is to check my ranges. It's pretty hard to know for sure how long things are without something to confirm it.

 

[chutestrate]

Thanks guys. I guess I'm one of those people who will over think this. I thought i was clear, but now I'm lost again.

thanks for trying to help. I have a feeling that a range finder will not be a good investment.

 

[jmr40]

A range finder can still be a very good investment. They make models that calculate the up or down angles and figure that into the range. If not just remember the range finder will over estimate the range shooting up or down. Unless the angle is extreme it usually doesn't really change your bullet drop enough to matter.

 

[rcmodel]

just remember the range finder will over estimate the range shooting up or down.

No, it actually will determine the exact range to the target you are ranging.

Regardless of the angle to the target.

All the angle compensating range finders do different is tell you the 'apparent' range factoring in the angle.

But as already mentioned earlier.

The problem is not the change in trajectory of a high velocity rifle bullet being effected much at all by an up or downhill ange in a normal hunting range shot.

The problem is getting the hunter to forget all the angle nonsense, and figure out where to hit the deer in the kill zone.

Shooting from a steep hill above, you will tend to hold too high on the back, and shoot over the heart/lungs.

Shooting uphill, you will tend to aim under the kill zone and shoot under it.

And no range-finder can compensate for that.

The hunter has to understand his perspective of the target from above or below it.
And aim where the kill zone actually is from that perspective.

Not where it appears to be if the animal was standing on flat level ground and the shooter was too.

And as others have said.
Angle is much more critical to a bow-hunter setting in a tree stand 25 feet up, and shooting almost straight down at a deer 20 yards away.

Then it is to a rifleman shooting from the side of a rolling hill into the valley 150 feet lower 300 yards away.

rc

 

[Captcurt]

Time of flight for an arrow can best be calculated with a calender....

True, so true.

 

[Joemyxplyx]

Witchhunter wrote:

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!

Mostly right. However, I saw a mountain goat hunter on TV overshoot a nice big ram at a rangefindered 350 yards twice. The problem was the goat was very high up a steep mountain so the horizontal distance wasn't very far.

Here's the calculation assuming a 60deg angle (not unusual in the Rockies for goats) and a 30-06 with a 165gr bullet at 2800fps.

horizontal distance = rangefindered distance (350 yds) x cosine 60deg (.5)

horizontal distance = 350 X 0.5 = 175 yards

So the drop in the 165gr 30-06 at 350 yards is 14.5" (with 200yd zero and other reasonable assumptions). However, at 175 yards he would have been an inch high not 14in low.

So, on flat ground, he would have held 14.5" high from his kill point on the goat to hit it at 350 yards. In the mountains, he should have held an inch low. He was shooting over 15 inches over his aim point which is why he shot over - twice.

A person used to flat ground would have held at the top of the goat; that's pretty instinctive. That would put him a foot or so over the goat. That's what it looked like on TV judging from the bullet splash in the snow.

Moral of the story, use a rangefinder that has an angle compensation built in when hunting in steep mountains. If you don't have one the rule of thumb is aim at the bottom of the animal at long distances when you have steep angles up or down. If you shoot low and aim on fur you should be good to go.

 

[brainwake]

Plus try and remember that while experiencing buck fever.

Hey there is actually skill in hunting. Sometimes you just have to miss before the lesson drives home.

 

[Joemyxplyx]

Brainwake wrote:

Plus try and remember that while experiencing buck fever.

I wrote that about mountain goats to show a practical example of the need to make a cosine correction. It really doesn't come up much apart from mountain goats. A 60deg slope is damned steep. While climbing up one, your hands will be close to the ground in front of you. Nothing much would venture going up something that steep except goats and goat hunters. Anything steeper and you would have to rope yourself off to climb it.

By way of comparison, stairs and roofs usually go between 20 and 40 degrees. I don't think they pave roads steeper than 25 degrees.

For more reasonable angles and hunting environments, the cosine correction is minor. The rule of thumb is shoot a bit lower than you normally would and don't shoot off the fur when shooting up or down hill.

BUT mountain goat hunting is probably the most physically demanding type of hunting there is. If I spent a week or two climbing up and down really steep mountains, sleeping huddled on rocky cliffs, enduring freezing cold and burning heat while gasping in thin air only to miss a shot at a trophy goat because of cosine error, I would be pissed for years.

Cosine correction is not really complicated, its just hard to visualize. I think of it this way - if you shoot straight up or down, there is no correction for bullet drop. If you shoot horizontally, bullet drop is at a maximum. If you shoot up or down at a 45 deg angle, the bullet drop is half of what it is horizontally.

Or visualize this: stand under a strong light and raise and lower your arm and watch the shadow. The shadow grows and shrinks as your arm raises and lowers. The length of the shadow is the length of your arm times the cosine of the angle of your arm from the horizontal.

If you think of the length of your arm as proportional to the shooting distance then the length of the shadow is proportional to the horizontal component the bullet drop is calculated from. As you raise your arm (or lower it) the shadow shrinks and the bullet drop calculated from the length of the shadow shrinks. That's all, that's it. No big deal. Now go out and shoot a mountain goat.

 

[brainwake]

For more reasonable angles and hunting environments, the cosine correction is minor. The rule of thumb is shoot a bit lower than you normally would and don't shoot off the fur when shooting up or down hill.

Minor for rifles...not minor for bows

But yeah, I think the OP was referring to rifles...so not much concern there.

 

[Andrew Leigh]

In trying to understand the math I took my Sierra Ballistic program and entered into it a 180gr. bullet with a MV of 2700. The rifle is zeroed at 100yds and I took the drop from 100 to 200yds which was 3.88". The time to travel from 100 to 200yds is according to them is 0.1233s. I then tried to replicate this using the formula 1/2g*t^2.

= 16 * 0.1233^2 = 0.2432' = 2.91"

Why can't I get to 3.88" as Sierra does. What am I doing wrong?

 

[MCMXI]

Why can't I get to 3.88" as Sierra does. What am I doing wrong?

MCgunner only gave you part of the relevant equation for displacement with constant acceleration. Since you're only concerned with bullet drop, acceleration in the vertical direction is constant due to gravity but your bullet has an initial velocity (Vo) in the -g direction at 100 yards. You need to add Vo*t where Vo is the velocity at 100 yards and t is the time of flight.