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There are a lot of theories and rules of thumb out
there about what to do when shooting up or down a hill. A person's natural
instinct typically causes them to compensate in the WRONG direction. When your
weapon is sighted in on level ground, your sights take
into account the trajectory of the bullet. As the bullet leaves the
barrel, it immediately begins to fall, as gravity acts upon it. In fact, a
classic physics experiment is to fire a bullet from a gun that is pointed level
to the ground and, at the same time, to drop another bullet from the same
height, but straight down. In this experiment, it is proven that both bullets
will hit the ground at exactly the same time. The forward movement of the bullet
is not relevant to the force of gravity that is acting upon it. How far the
bullet goes is strictly a factor of how fast it is going and how far off the
ground it was when it was launched. As an example, if the bullet is fired from a
height that takes 1 second for it to hit the ground, then we can calculate the
distance it will travel over level ground before it impacts. A bullet traveling
1000 feet per second will go 1000 feet, a bullet traveling 3000 feet per second
will go 3000 feet and a bullet with zero forward velocity will go zero feet. But
all will hit the ground in 1 second.
When rifles are sighted in at some distance, say
100 yards, the sights are being set up to factor in
this bullet drop. So what happens is that when you are looking
through a sight, directly at the target over level ground, your barrel is
actually pointed slightly upward. As a result, the bullet travels in an arc that
can be plotted as traveling below the extended line of the barrel, yet above the
line of sight. After its descent, it lands on the target which is at the line of
sight. The force of gravity on that bullet is constant throughout its
 arc.
In our diagram, we can see the center section
where the line of sight is level, the barrel is aimed slightly upward as shown
by the slight rise of the bullet after firing. Then, since the sight and barrel
are properly adjusted, the bullet impacts at the point of aim even though the
line of sight and bullet travel path are different.
When shooting uphill
things change. Your line of sight seems to be the same but, since it
is at an angle, the actual distance is greater. Since your barrel is already
pointing upward when sighted in on level ground, shooting uphill points it even
further upward. This results in the trajectory of the bullet to be over the
target as shown in the upper part of the diagram. If you aim your weapon which
has been sighted on level ground directly at the target the bullet will probably
sail harmlessly over the top of the target. Of course, the actual angle and
distance matter. If the angle and distance is small enough, the difference will
be minimal and you may still strike the target. However, as distance gets
greater and the angle increases the odds of hitting even a large animal sized
target diminish.
Therefore, when shooting
uphill, you have to hold your sights below the target in order to hit the
target. Many people assume the opposite and end up shooting right
over their target.
A similar effect occurs
when shooting downhill. Your line of sight is angled downward and so
is your barrel. However, your barrel is still pointing higher than the line of
sight since it was zeroed for level ground. The net result is the same as
shooting uphill. The bullet will overshoot the target as shown in the lower part
of the diagram. To compensate for shooting downhill you
must also aim below the target in order to have the bullet
strike. Assuming the angles and distance are the same the bullet will miss the
target by the same amount regardless of whether you are shooting uphill or
shooting downhill.
In all cases, gravity acts upon the bullet the
same way and the drop rate is always the same. The drop rate is defined by the
acceleration rate of gravity on earth, which is always 32 feet/second2.
The confusing result is that whether you're
aiming up or down, you should always aim low. This is due to the fact that the
barrel and the sight are never in line with one another. When shooting at an
angle, the actual distance is always longer, making the bullet not line up with
the line of sight as you've adjusted it to on level ground. |
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