Hunting Soon!

[RobRez 1,895]

So, I hunt Deer and Elk on pretty steep terrain in the mountains of Western Montana. I "heard from a guy who heard from a guy" that bullets don't "drop" the same when shooting up or downhill. I've not really noticed this, myself. Is this true, and if so, how much of a difference are we really talking about? Is it enough to make you miss a shot at say, 200 yards? I'll be shooting a bolt action .308.

[banshee 69]

here are a couple of articles on this

Up and Down Shooting

 

When you shoot uphill, you aim over the target, and if the target is downhill, you hold low. Or is it the other way around? Or does it make any difference? Shooters love to argue this, but seldom do we hear a convincing argument or explanation. The answer is easy once you understand the principle. Before getting into reasons why, let me promise you that the point of aim does indeed change if you shoot at a target either above or below you. Also, as the uphill or downhill angle increases, so does on-target bullet displacement.

 

Let's say you're going after a mountain goat with your new rifle in .270 Winchester caliber. You adjust the riflescope so that the bullet hits dead center at 200 yards. This means that the point of impact is 1 1/2 inches high at 100 yards, about 6 1/2 inches low at 300 yards and 19 1/2 inches low at 400 yards. In the mountains, you get a shot at a big billy at what you and your guide estimate to be close to 400 yards. It's a longish shot, but you roll your down vest into a rifle cushion and, taking a solid rest, settle the crosshairs a few inches over the goat's back. Allowing for the 19 1/2 inch drop at 400 yards, you hold high and figure that the bullet's trajectory will curve right into the goat's boiler room. But you have forgotten something.

 

The goat is 45 degrees above you. What a difference will that make in the bullet's flight in respect to the line of sight? Do you need to aim higher, perhaps? Or lower?

 

If you aimed at the goat the same way that you would over level ground, the old billy would live to see another winter because the bullet would zip harmlessly over its back. At an upward angle of 45 degrees, the bullet would only fall 8 1/2 inches in respect to line of sight, less than half as much as it would drop when shot horizontally. Exacly the same would be true if you were shooting downhill at a 45 degree angle.

 

By now, we're very much aware that gravity causes the path of the bullet to curve downward. But let's put gravity to work another way. Imagine, if you will, a rifle held perfectly perpendicular so that when it is fired, the bullet goes straight up. If there were no wind or other variables to disturb the bullet's flight, the projectile would go straight up until it ran out of momenum. Then, gravity would pull it straight back down again, base first.

 

Here's where you need to use your imagination. This time, we'll tilt the rifle slightly, say 1 degree off plumb. This time, the bullet's flight is almost straight, but not quite. As it nears the apex of its trip, it begins to curve ever so slightly. So, let's keep shooting (and using our imaginations). Each time we fire, we tilt the muzzle another degree from the vertical, and the bullet path of each succeeding shot is progressively more curved. At last, we come to 90 degrees (horizontal), and the bullet follows its normal curved trajectory.

 

By now, I'm sure that you're getting the picture. we know that our logic is sound, but it must be demonstrated. How? Let's use an ordinary fly rod- the one you take trout fishing! Assuming that the rod is not warped, set the butt on the ground and hold the rod vertically. It remains perfectly straight. Now, holding it by the grip, lower it horizontally. See how it curves downward? (If it is too stiff to bend very much, attach a weight to the tip.) Now, slowly angling the rod from the vertical to dead level, note how gravity causes the curve to increase as the angle nears the horizontal.

 

Now, let's get down to basics. Realizing that shooting uphill or downhill causes the bullet to strike higher than it would on the horizontal, in respect to point of aim, how can we judge where to aim? The easiest way is to use a cartridge with a flat trajectory. Obviously, if a bullet has a relatively little cuve in its trajectory, that curve will be less affected by variations in shooting angle. For example, let's compare two cartridges with widely different trajectories. One is a 7mm Magnum with a 145-grain pointed bullet loaded to 3,100 fps. The other is a .30/30 Winchester with a 150-grain flat-nose slug loaded to 2,100 fps. Both rifles are scope-sighted, with the scopes 1 1/2 inches above the bore centers, and both are sighted in at 200 yards.

 

Changing the angle form horizontal to 45 degrees but still shooting at 200 yards, the 7mm Magnum hits 2 32/100 inches above the point of aim, and the .30/30 is all of 5 65/100 inches high. Now you see why a cartridge with a flat trajectory elimnates some of the guesswork when shooting uphill or downhill.

 

Another way of "guesstimating" where to hold on angle shots is my rule of thumb. For many years, I've reasoned that a bullet fired at an upward or downward angle will hit very close to where it would hit if fired over the horizontal leg of the angle-the horizontal distance to a vertical line running through the target.

 

A triangle is formed by the horizontal line, the steep (angled) line from rifle to target, and the vertical line from the horizontal distance up to the target. In any such triangle, the horizontal line is shorter than the uphill (or downhill) line. Gravity affects the bullet's flight only for as long as the projectile is in the air - along the shorter, horizontal line.

 

By Jim Carmichel from the 1991 Outdoor Life Deer Hunter's Yearbook

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here is another

 

UPHILL/DOWNHILL SHOOTING REVISITED

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.

 

Barrel-Scope Relationship

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!

 

 

http://www.wildsheep.org/magazines/article...ll_shooting.htm

Edited September 9, 2009 by Banshee

[RobRez 1,895]

Wow! F**cking awesome information!!! I had no idea the effect was so great. coming to this site is like having a Giant Brain!!

Thanks a lot!

[zenmetsu 17]

yeah, he beat me to it.

 

Another way to look at it is... if you were aiming at an enemy helicopter 500meters directly overhead, you would not "hold high" since gravity will be pulling straight back at you... literally this would be like shooting point-blank since there is no horizontal distance being traversed by the bullet.