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Carry-a-Kimber wrote:The easier math is a squared + b squared = c squared. A being your height and B being straight line distance from grade to target. So if you were 200 yards at grade and 20 yards above the target the line of sight distance would be 200x200 + 20x20 or 40,000 + 400 = sight distance of the square root of 40,400 which is 200.99 yards. You have to be way above the target to make a real distance.

Yeah, a tree stand doesn't make much difference. But I've hunted where I was up on a hillside shooting down into a valley. Have a vertical distance of 300-400 feet, and that can matter.

It is also critical to be aware of incline shots if someone is a bow hunter, because an arrow trajectory is more much severe than a bullet trajectory. Shoot downhill from a tree stand and you can put an arrow in the dirt if you aren't taking it into account.

AndyC wrote:

Jumping Frog wrote:

AndyC wrote:You calculate drop over the horizontal distance - calculate the true horizontal distance (because gravity affects bullets only straight down) using cosine tables

This is a good approximation, but it is not the complete answer.

No, it's a pretty accurate rule of thumb called the Rifleman's Rule and is accepted in the sniper community world-wide;

Since you have obviously thought about this topic, you may be interested in an article called "Incline Shooting" in the current (Jan 12) issue of GUNS magazine (page 52).

Digital edition available here (free but requires an email address): http://fmgpublications.ipaperus.com/FMG ... /GUNS0112/" onclick="window.open(this.href);return false;

The article addresses the cosign method first. Using a classic .308 Winchester target load with 100 yard zero, the horizontal elevation adjustment needed at 500 yards is 63.3". At a 40 degree incline -- which calculates to a 383 yard horizontal distance -- a 31.1" adjustment is calculated. The exact elevation adjustment needed for a 40 degree angle shot at 500 yards is 43.6". Thus, the error using the cosign method ("Rifleman's Rule") is 31.1 - 43.6 = -12.5". Not too good if you are trying to make a hit on an 8" kill zone.

It concludes:

This method is correct in reducing the effective distance to the target. However, the bullet will spend just as much time getting to a 500-yard target at 40 degrees as it will getting to a target that is horizontal. That is because gravity pulls the bullet's trajectory over 500 yards, not 383 yards. The ... drop value is based primarily on the gravitational force over the time of flight.

The second rule of thumb method uses MOA instead of inches. Subtract 1 MOA for every 20 degrees. A 500 yard horizontal shot takes a dial comeup of 12 MOA from a 100 yard zero. Using this method, the hunter looks at the chart, see 12 MOA, and then subtracts 2 MOA for the 40 degrees (1 MOA per 20 degrees). The end result is a 10 MOA comeup. However, the true correction for 500 yards is to comeup 8.5 MOA. A 1.5 MOA error at 500 yards translates to a 7" error, so the result is slightly better than the cosign approach.

A better approximation was developed at Perry-Systems It basically multiplies the horizontal comeups (for the actual target distance) by the cosine of the incline angle and subtract a bias value. This method takes the actual time of flight into account.

AndyC wrote:You calculate drop over the horizontal distance - calculate the true horizontal distance (because gravity affects bullets only straight down) using cosine tables

This is a good approximation, but it is not the complete answer.

Using your example, the actual bullet drop will be greater than the bullet drop for a "pure" 459.6 yard horizontal shot.

Why? Because the trigonometry does not take wind resistance into account. A bullet fired horizontally for 459.6 yards will travel approximately the same distance. However, firing up or down at 40 degrees means the bullet will still travel 600 yards total distance (the hypotenuse), thus increasing the amount of time in the air and resulting in a slower velocity at impact with a greater bullet drop (due to longer time in flight).

How much lower? That is entirely empirical, depending upon the bullet's ballistic coefficient, air pressure/temperature, wind speed etc. But for long distance shooting, the difference is definitely measurable.