Aim Above or Below?

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huntersjournal
 
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Aim Above or Below?

Postby huntersjournal » Sat Oct 30, 2010 1:50 pm

A controversial issue

If the target is not at the same level as the shooter, should he aim the rifle above or below the critical point?

As you can see in our (exaggerated) graphics, when you aim horizontally using a properly calibrated scope, the barrel of the rifle tilts slightly upwards to compensate for the parabolic effect. In other words, the bullet crosses the sight line immediately after you pull the trigger and travels above that line most of the time.

If you follow our sight-in adjustment guidelines, as described in another article, both lines will intersect at the vital zone at about 250 to 300 yards. For a target located about 100 yards, the bullet will hit 3 inches above the crosshairs’ center.

However, what happens when you are lying on the top of a hill and the game moves in the valley below? What if it is you in the valley and a sovereign reindeer roams a mound?

Here is a typical story:

A hunter, standing at the foot of a hill sees a white-tailed doe on the top. He focuses slightly behind the shoulder bone of the deer, fires and… The female deer flees as the bullet raises dust behind her. Missed miserably. But, why?

To understand the reason we must consider which factors affect the trajectory of a bullet. These are:

• Thrust
• Rotation
• Nutation (pitch)
• Air resistance
• Gravity
• Mass
• Shape of the bullet
• Initial velocity
• Length

(Actually, there are only two important factors: the ballistic coefficient of the bullet and its velocity.)

Modify one of these factors and the outcome will vary as well. For instance, if you adjust the scope for a cartridge that leaves the muzzle at 1063 yards per second and then use another cartridge that develops an initial velocity of 890 yards per second, the impact at 250 yards will hit about 4 inches below.

Normally, when we sight-in a rifle both target and barrel are more or less at the same level.

However, when shooting upwards, we are modifying several factors at once: gravity and air friction, among others. A bullet fired upwards tends to slow down due to gravity. Conversely, if fired downwards, it tends to accelerate.

To counteract the effect, always aim below the desired point when shooting upwards or downwards

In real life, the effect becomes important when the angle between the target, gun and the horizontal line across the muzzle exceeds 30 degrees.

Now, by way of entertainment, consider this ballistic question:

If you fire horizontally and drop another bullet with similar characteristics from the same height, which one reaches the ground first?
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kellory
 
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Re: Aim Above or Below?

Postby kellory » Wed Jul 27, 2011 8:31 pm

Both bullets strike the ground at the same instant because gravity is a constant on two objects of the same mass and shape.
The only real difference between a good tracker and a bad tracker is observation. All the same data is present for both. The rest is understanding what you are seeing.

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mnslayer
 
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Re: Aim Above or Below?

Postby mnslayer » Fri Jul 29, 2011 3:06 pm

SO IF YOU DROP A FOOTBALL FROM SHOULDER HIEGHT IT WOULD HIT THE GROUND AT THE SAME TIME AS ONE THROWN AT SHOULDER HIEGHT. WHERE IS THE EFFECT OF ROTATION FROM THE RIFLING. THIS CAUSES BULLETS AND( FOOTBALL THROWN IN A SPIRAL ) TO TRAVEL FURTHER AND MORE ACCURATE THEN A SMOOTH BORE GUN OR A NON SPIRAL THROW OF A FOOTBALL. SOUNDS LIKE THE LOGIC IS A LITTLE FLAWWED HERE.

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kellory
 
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Re: Aim Above or Below?

Postby kellory » Sat Jul 30, 2011 6:28 am

No, they hit the ground at the same time. A football goes further due to the fact it is thrown upward. Both ball and bullet spiral to prevent wobble and increase air resistance. 45 degrees will give you maximum range. But a hammer and a feather fall at the same rate on the moon. If air resistance is the same on both objects, and because gravity is a constant, if fired level (not upward) both should strike at the same instant. Because your gun can not fire at escape velocity, and because the rise and fall of the bullet cancel out, Gravity is a constant. I am no physics professor, but dad was an engineer,and a hunter, and we discussed that very thing many years ago. If we missed something, by all means enlighten me. Your scope sees line of sight, but your bullet travels in an upward arc, to intercept your scope at a preset, and predictable distance. Parallax is the difference between the two paths.
The only real difference between a good tracker and a bad tracker is observation. All the same data is present for both. The rest is understanding what you are seeing.

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mnslayer
 
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Re: Aim Above or Below?

Postby mnslayer » Sat Jul 30, 2011 1:01 pm

you fail to take in account that velocity creates lift. if gravity and wieght were the only factor all 150 grain bullets would shoot the same,yet we know boattails,pointed and round nose bullets all hit at different hieghts from a rifle that is benched mounted and not adjusted for each bullet.

Dan Salmon
 
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Re: Aim Above or Below?

Postby Dan Salmon » Sat Jul 30, 2011 1:20 pm

They drop more or less than each other at a given distance because of velocity generated or conserved by the bullet design, not because they are defying gravity more than the other. Different bullet designs don't negate gravity, they negate other factors such as drag, etc. which allows more efficiency in the horizontal direction. This efficiency allows a bullet to reach a given distance faster and thus on a flatter trajectory related to gravity at a given distance than a less efficient bullet design.

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kellory
 
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Re: Aim Above or Below?

Postby kellory » Sat Jul 30, 2011 1:41 pm

Again we are talking about air resistance. different shaped bullets cut through the resistance differently. just as a different powder charge affects velocity, a different shape impedes it. A rifled slug would tumble without the rifling om the slug to give it twist, increasing its surface to the resistance and decreasing it's speed. An air plane would never take off without the lifting effect of the uneven shape of the wing, creating lift. I do not believe a bullet can have a lifting effect, because the shape is symmetrical. No matter how sharp the point, the lift and the drop would evenly match. A flat nosed wad cutter would match up very poorly with Teflon coated or fully jacketed bullet, due to air resistance. All things being even, weight, distance, powder charge and wind. If I have missed anything, please explain, This is my understanding., but I am not not perfect! enlighten me. Two bullets of the same resistance to air resistance, one dropped without tumble, one fired level, no wind. They should strike together.
The only real difference between a good tracker and a bad tracker is observation. All the same data is present for both. The rest is understanding what you are seeing.

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mnslayer
 
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Re: Aim Above or Below?

Postby mnslayer » Sat Jul 30, 2011 4:39 pm

Want to know where to aim go to your hunting area and set targets on hills and valleys and see where your gun is hitting. proof positive without the what ifs of the science. from experience hunting shots taken at extreme angles for me always seem to hit low of my aim point. the truth is you should be holding lower then normal for either shot up or down. still would like to try the shots for real and see what affect it has on my cartridge 270win.

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kellory
 
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Re: Aim Above or Below?

Postby kellory » Sat Jul 30, 2011 6:25 pm

I agree, but this was a balistics/logic question....just for fun. However, with that said.....always know the how and the why of anything, if you can. It could save your life, or the life of someone else. Same with plants, food/poison, medical or seasonal. It pays to know what it is, and how can it be used. morel/toadstool?
The only real difference between a good tracker and a bad tracker is observation. All the same data is present for both. The rest is understanding what you are seeing.

pwbsmokey
 
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Re: Aim Above or Below?

Postby pwbsmokey » Sat Sep 17, 2011 8:36 pm

When shooting uphill or downhill gravity has less effect on a bullet, and it will strike high, so aim low.


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