Help on shooting from a tower.....

[MCMXI]

When shooting up or down a steep ravine or hill, you have to determine the "level" distance between you to the target. This is why a bullet appears to hit "high" when fired uphill or downhill. The "level" distance is the distance that gravity acts on the bullet.

This is incorrect. The reason why a bullet drops less when fired at a target at an angle other than 0˚ and at the SAME distance away is that the component of g (gravity) perpendicular to the path of the bullet is LESS than g. For example, at an angle of 45˚, the vector component of g that is causing the bullet to drop relative to the point of aim is 0.707g. The figure below shows how to calculate the value of g using a 20˚ angle as an example. For all intents and purposes, a bullet's time of flight to a target X yards away at 0˚ is the same as the time of flight to the target X yards away at an angle of 45˚. Given enough decimal places this can't be true since the bullet fired horizontally has no g component acting opposite to the direction of the bullet's path. A bullet fired at a positive angle does have a component of g that is opposite to the direction of the bullet's path, however, velocity decrease due to -g is negligible compared to the bullet's velocity within practical ranges.

For example, consider an extreme but plausible situation encountered by a hunter. The hunter ranges a deer at 424 yards at an angle of +45˚. Against the advice of Sierra, he's using Federal Gold Medal Match 168gr BTHP ammunition. He has a ballistic table for FGMM 168gr (0˚) printed out in 25 yard increments taped to his stock. He looks at his table and sees that the bullet drop at 425 yards is 42.0". He remembers your post and you saying "The "level" distance is the distance that gravity acts on the bullet" [when shooting uphill or downhill] so he does a quick calculation using x = r*cosθ where x is the horizontal distance to the target (what you refer to as A-C) and r is the distance that he ranged (what you refer to as A-B). He calculates that the horizontal distance is 300 yards. So he looks at his ballistic table for the FGMM load and sees that with a 100 yard zero at an angle of 0˚ the bullet drop at 300 yards is 16.0". If he uses a 16" holdover, his bullet will hit 9" below what he's aiming at because the drop at 425 yards at +45˚is 25.0" and not 16.0".

The easiest way for a shooter to make corrections for long-range shots that are at significant positive or negative angles is to have a cosine indicator installed on their scope such as the one shown below. The indicator gives the shooter a number which is multiplied by the bullet drop at 0˚ at the range of interest. I don't know if the cosine indicator is as good as a ballistic table or if it simply gives a good enough estimate. In the example above, .707 * 42.0" = 29.7" whereas ExBal calculates the bullet drop to be 25.0". So merely multiplying the cosine of the shooting angle by bullet drop at 0˚ at the range of interest omits some variables.

 

[Mike J]

How did people ever manage to kill deer before calculators & computer gizmo's?

For whatever it's worth I think Art has it right.

 

[bobleeswagger]

Evidently at relatively close ranges and insignificant angles

 

[Art Eatman]

Well, too many one-shot kills at various distances and angles over some thirty-five years of very-active deer hunting for me not to believe what I said.

My 500-yard target is about 250 feet lower than my shooting table. The rifle thinks it's about 493 yards. (Cos = 0.986, about 9.5º). But I generally won't take a 500-yard shot on a deer, and inside of 300 it just doesn't matter.

 

[MCMXI]

How did people ever manage to kill deer before calculators & computer gizmo's?

How many deer have been missed or maimed due to hunter ignorance and or incompetence. Maximum point blank range has been used effectively for years and this is what Art is describing. It's a simple and effective method for many situations. However, I made the post above simply to provide an accurate explanation of the reason why a bullet drops less at positive or negative angles. It's up to the individual to decide whether or not that information is useful to them. Frankly, I'd rather have all the facts and make an educated decision.

 

[chrt396]

If you have an iPhone, there's probably an app for that.

actually..there is!! Winchester bullistics!

 

[kludge]

I'd advocate sighting in for two inches high at 100 yards. That's dead-on at 200 and about six inches low at 300. Saves a lot of worryin' and thinkin'. "Point it and pull; hell ain't half full."

Any angle less than twenty degrees or so needs no correction, up or down, for distances under 300 yards. Even at 45 degrees, the rifle thinks it's only 70% of the sight-line distance. Inside 200 yards? "Point it and pull."

Look at all the worryin' I saved ya.

^this^

 

[KodiakBeer]

I'll give you a trick that makes MPBR shooting pretty foolproof.

Nearly all rifle scopes use some sort of duplex reticle. In effect, you have a thicker portion near the edges, and a fine line near the center. You can use these as an easy ranging indicator.

At 100 yards adjust your variable scope from lowest to highest until you can bracket a 6" bullseye between the fine horizontal crosshair and thick part of the vertical crosshair on the bottom. On most 3x9 scopes that bracket will be at about 6 power, yours may be 5 or 7 (or whatever), but just remember at what power your duplex can bracket a 6" bullseye at 100 yards.

Now, a deers chest averages about 18" from top to bottom. That's 3X the size of the 6" bullseye, so it will bracket a deers chest at 300 yards. Lay the horizontal crosshair across the deers back and adjust the lower "fat" part of the reticle until brackets the bottom part of the chest. Check the power of the scope when you get the bracket. If the power is higher than your bracket for the 6" bull, then the deer is beyond 300 yards and you need to get closer.

If the setting is lower, then then the deer is closer than 300 yards so just shoot.

If the deer is exactly bracketed then he's right at 300 yards, so (depending on what you're shooting), you might want to hold slightly high, perhaps on the top half of the chest, and shoot.