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Aim Above Or Below?


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#1 huntersjournal

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Posted 30 October 2010 - 01:05 PM

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?

#2 PA RIDGE RUNNER

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Posted 30 October 2010 - 06:05 PM

Good article. Oh btw both hit the ground at the same time.
If God had a refrigerator would your picture be on it.
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#3 Leo

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Posted 31 October 2010 - 02:51 PM

Yep! Shooting uphill - Aim low. Shooting downhill - Aim low!

PA is right the bullets hit the ground at the same time. :)
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