Fieldcraft Down Hill/Shooting from Above

[2manygunz]

I'm used to range fire so have never really encountered shooting down on a target at a distance, until this weekend, when I shot at targets that spanned hills. 200 yds was a straight shot while 100 yds was in the hollow.

I was surprised by how big of a difference the downhill angle makes. At 100 yds, where my site is zeroed, I was consistently high. My first three rounds, in fact, nicked the very top of the paper target.

Newbie question, I'm sure, however are their any general rules of thumb when it comes to shooting down on a target? All I seemed to learn from the brief experience at this site was that holding at zero while shooting downhill will leave your shots high.

 

[SurgeonPredator]

Re: Down Hill/Shooting from Above

If you want to go full mathematics: Link

Get a cosine level indicator to attach to your scope/rifle if you have alot of inclined shots to take.

 

[benjaminzero]

Re: Down Hill/Shooting from Above

It's the same for shooting uphill. Shooting at any angle will always cause your rounds to impact high.

There are formulas to correct for this, and some rangefinders and ballistic software will correct for this but I don't have either handy. A search will probably pull it up, look for angle shooting.

Also, at 100 yards, no angle would cause you to miss more than about an inch, there may be more factors at play.

 

[desertHK]

Re: Down Hill/Shooting from Above

Up, or down hill, Aim low always. Assuming that you zero your rifle at 100 yards on a flat surface, when shooting up/down hill, you need to know the angle of inclination. Get the Cosine of that angle and multiply by the range will give you the "true" range, base on your horizontal zero. Then it's a matter of looking up the DOPE.

 

[Lindy]

Re: Down Hill/Shooting from Above

Multiplying the actual range times the cosine of the angle, and then using the dope for that range is called the Rifleman's Rule - and it will not work for shots at long range and significant angles. The Rifleman's Rule was used when snipers shot at typically short ranges with angled shots.

The Improved Rifleman's Rule is to multiply your <span style="font-weight: bold">dope</span> for the actual range by the cosine of the angle. It's just as easy to do, and significantly more accurate.

If you want to know why, see this reference from Sierra Ballistics:

Inclined Fire by William McDonald

 

[2manygunz]

Re: Down Hill/Shooting from Above

Excellent info. Thanks everyone. Knew this was well-trodden ground. So, to summarize:

math = angle, cosine and multiply
gut = "aim a little low"

To the comment about being 1" or so high. I was probably 2" high and it was a small paper target. Further, I'd say my true zero is probably a 1/2" high overall at 100 yds. It's an aimpoint reddot 2MOA, so between the lax zero, small target and dot size, I'm not surprised I ended up so high.

 

[KNIGHT11B4]

Re: Down Hill/Shooting from Above

One tricky part...

Remember that if you are using your optics to determine the range (miling) that in order to use the formula above you must mil the target horizontally.

If you mil vertically you are miling a false apperance of the object due to the fact that you are up or down hill of the object.

Horizontal Mil-> range x cosine = Flat Ground Distance

Verticle Mil-> range x cosine x cosine = Flat Ground Distance

 

[2manygunz]

Re: Down Hill/Shooting from Above

Man, they make this stuff look easy on xBox.

 

[desertHK]

Re: Down Hill/Shooting from Above

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lindy</div><div class="ubbcode-body">Multiplying the actual range times the cosine of the angle, and then using the dope for that range is called the Rifleman's Rule - and it will not work for shots at long range and significant angles. The Rifleman's Rule was used when snipers shot at typically short ranges with angled shots.

The Improved Rifleman's Rule is to multiply your <span style="font-weight: bold">dope</span> for the actual range by the cosine of the angle. It's just as easy to do, and significantly more accurate.

If you want to know why, see this reference from Sierra Ballistics:

Inclined Fire by William McDonald </div></div>

Thanks Lindy.

 

[SierraOne1]

Re: Down Hill/Shooting from Above

That's the first I've heard of that, Lindy. Thanks, man! (again, haha)

 

[SierraOne1]

Re: Down Hill/Shooting from Above

Lindy:

Just to be clear: So you're taking the adjusted measurement (after cosign multiplication), then your dope from that computation, and then multiplying by the cosign AGAIN? Can that be right? Or do you just multiply your LOS measurement's dope by the cosign?

 

[Lindy]

Re: Down Hill/Shooting from Above

Here's an example:

Actual range to the target is 800 yards, and the angle is 30 degrees.

My dope for an 800 yard shot is 8 mils.

The cosine of 30 degrees is 0.866

8 mils * 0.866 = 6.9 mils.

If I do it the old way, multiplying the range by the cosine, I get 692 yards. My dope for a shot at that range is 6.3 mils.

The difference is 0.6 mils, and 0.6 mils at 800 yards is 17.3 inches. That's a big difference on a target.

 

[SierraOne1]

Re: Down Hill/Shooting from Above

Rgr that. Thanks for the clarification.

 

[ggmanning]

Re: Down Hill/Shooting from Above

That is interesting Lindy. What conversion error are you eliminating by decreasing the dope vs decreasing the range and using the dope for the new range?

 

[ggmanning]

Re: Down Hill/Shooting from Above

Are you saying if mils are used to range vs a rangefinder to muliply the esimated range by cosine for your angle 2 times?

So...I get a range estimation of 600 yards using my mils....at 45* I would use 600 X cosine for 45* X cosine for 45* to get true horizontal distance to target.

Is that to factor both angle of shot and angle on target while mil-dot ranging?