UPHILL/DOWNHILL SHOOTING REVISITED
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In the Winter 2001 issue of GRAND SLAM, we featured an article about
uphill/downhill shooting. The article was a collection of information
by various GSC/Ovis members. There is still some confusion out there as
to what is really involved in uphill/downhill shots, so here is a theory
recap of angle shooting and how to prepare yourself for the shot.
Gravity From a Shooter's Perspective
First, the context of what is meant by "gravity" in this article
needs to be clarified. Of course, gravity affects all things on earth.
If you shoot a bullet straight up in the air, it is going to come down
because of gravity. If you shoot a bullet across level ground, it is eventually
going to land somewhere because of gravity. In the scientific sense, gravity
always affects a bullet.
However, for this article, think of gravity only as it affects the "curve"
or "arc" of bullet's path. More gravity will produce a path
that curves downward more, less gravity will produce a straighter or "flatter"
bullet path. The other aspects caused by gravity, such as time of flight,
are insignificant when applied to exterior ballistics.
To understand uphill/downhill shooting, you must first understand that
your line of sight and barrel are not parallel. If you are like most rifle
hunters, you sight your gun in to shoot several inches high at 100 yards.
To accomplish this, your barrel must point higher than your scope’s
line of sight. As the distance increases, your barrel will point proportionally
higher and higher. The figure below illustrates this:
Using the bullet and load table at the bottom of the page as an example,
to achieve a 300-yard zero, the barrel must point almost 18” high
at 300 yards. Why? Because at 300 yards, gravity is going to push the
bullet down about 18”. Thus, you will be zeroed at that distance.
At 500 yards, the barrel actually points about 31" higher than the
line of sight.
Horizontal Distance is What Matters
By now, most people understand that gravity only affects the horizontal
component of a bullet's path--whether or not you are shooting uphill or
downhill. The steeper the shooting angle becomes, the less the horizontal
component of the angle becomes, and the less chance gravity has to "curve"
or "arc" a bullet's path. The examples below illustrate the
horizontal components of an uphill or downhill shot:
The less chance gravity has to affect your bullet (i.e. less horizontal
distance traveled), the flatter your trajectory becomes. In other words,
your bullet's path is closer to where you barrel is actually pointed.
By now you might be thinking, "Ok, so at 500 yards at a 60-degree
angle, my bullet is affected by gravity just the same as 250 yards on
flat ground, so I would aim like it was a level 250 yard shot, right?"
WRONG! This is the a myth in uphill/downhill shooting. The reason is very
simple. Forget about gravity for a moment, remember, you are actually
shooting at 500 yards and your barrel is pointed much higher from your
line of sight at 500 yards than 250 yards.
In order to accurately compute the uphill/downhill trajectory, we must
factor in BOTH the shot angle and the barrel-scope relationship.
Putting it All Together
Now that we understand the factors of angle shooting, how do we figure
out the numbers for our individual gun, load, and zero? The answer: A
ballistics program that computes uphill-downhill bullet paths at different
angles. There is no other way. You can either purchase custom made cards
for your particular load (check out www.ballisticards.com) or, if you
have a computer, you can purchase a ballistics program. Sierra Bullet's
Infinity ballistic program is excellent for this purpose. Be careful,
as some other programs will give you the angle measurement is Minutes-of-Angle
(MOA), and the only way you can use that data is to manually adjust the
dials on your scope. The way you use a ballistic program is to set up
the load and zero parameters for level ground, then see how the trajectory
looks at different angles, keeping those same load and zero settings.
Once you run all the numbers, you need to make a "cheat sheet"
and keep it with you in the field.
Measuring Uphill-Downhill Angle
On high-angle shots, accurate angle measurement is critical. There are
many tools available, with several being mentioned in the Winter 2001
issue of GRAND SLAM. There is one more tool that should be mentioned,
and it is called the Slope Doper. It looks like a protractor, but the
scale is positioned differently to allow you to sight down the straight
edge. A wire hangs down by gravity and will give you a good estimation
of the angle.
NOTE: The decimal numbers on the Slope Doper are for MOA calculations--if
you adjust your scope dials when hunting you would use these; if you use
the "hold over" method, you would ignore these and just look
at the degree measurement.
Reading Between the Lines
Does angle make a difference? You bet! One of the first things you will
notice when you build your "cheat sheet" is that your bullet
will be shooting above your point of aim at high angles--even at very
long ranges. Instead of our example bullet dropping almost 4 feet at 600
yards, the bullet now shoots high by over two feet at an 85-degree angle.
That's a difference of six feet! How could it shoot higher than your line
of sight? Remember, your barrel is pointed much higher than your line
of sight. To be more specific, our example barrel points about 31"
high at 500 yards. So, at an 85-degree angle (only a little bit of gravity
affecting the "arc"), it makes sense that you would shoot about
25 inches high.
Also notice that at high angles, accurate angle estimation is crucial.
Say you are hunting desert sheep and a big ram is below you at 400 yards.
The ram looks nearly straight down and you think the angle is 75 degrees.
Well, if the angle was actually only 60 degrees, you would have missed
your point of aim by 8 inches.
Remember the myth that you can just "triangulate" the horizontal
distance and aim as if you were shooting at that distance? Look at the
trajectory table (opened in a new window). At a 60-degree angle at 400
yards, our bullet strikes 7.3" high. If the myth was true, the 400
yard 60-degree angle trajectory would equal the 200 yard level ground
trajectory. Comparing the two numbers we see that our bullet shoots 3.7
inches higher on the 60-angle shot. That may not seem like much, but why
not be right the first time around? At further ranges or higher angles,
the difference becomes even more significant.
Take a moment to study the chart (opened when this page was loaded).
Even if you shoot a completely different caliber or load, the trajectory
is going to look fairly similar. There are more charts posted below for
you to reference. Good luck and good shooting!
More Uphill-Downhill Charts:
.264 120 gr.
J-36 Bullet @ 3,260 fps, zeroed at 300 yards
180 gr. Nosler Partition @ 2,900 fps, zeroed at 273 yards
225 gr. Barnes X @ 3,250 fps, zeroed at 200 yards
175 gr. Rem. Core-Lokt @ 2,860 fps, zeroed at 200 METERS (metric data)