# How close is the cosign rule in slope shooting?

#### newtonian

##### Active Member
I know of a great park where elk appear before dark. Like many such places the convexity of the open area makes it unlikely to see elk when on the ground there. I found a cliff vantage point that would offer shots from 250 to about 600 yards. The downslope is 21 to 26 degrees. For me, the easiest method is to just apply the cosign rule. My question is whether you have found it reliable enough for steep shots at fairly long ranges. It is surprisingly hard to find a place to test this out.

#### elkaholic

It will work fine at that distance!

#### dfanonymous

##### Well-Known Member
I know of a great park where elk appear before dark. Like many such places the convexity of the open area makes it unlikely to see elk when on the ground there. I found a cliff vantage point that would offer shots from 250 to about 600 yards. The downslope is 21 to 26 degrees. For me, the easiest method is to just apply the cosign rule. My question is whether you have found it reliable enough for steep shots at fairly long ranges. It is surprisingly hard to find a place to test this out.

If you’re using a kestrel w/AB just use cosine and plug it in. It will do all the hard stuff. If you using something else especially paper dope and you are at 20+ degrees, you can do your manual slope dope correction (cosine x distance) which is better than nothing. I wouldn’t say 600 is far, but as said, it will work fine.

#### Dog Rocket

##### Well-Known Member
If you apply the cosine to the distance, it go awry pretty quick.

Applying the cosine to your dope as a percentage gets much better results.

Ex: 500 yards @30* down slope. Cosine .866.

Don't apply the cosine of .866 to the 500 yards. If your dope is say, 10 moa for 500 yards, then 10 moa x .866 = 8.6 moa

#### newtonian

##### Active Member
Thanks; I have never run across that idea, but it makes sense. It has always seemed that the cosine rule is a bit too simple when you consider the changing quantities in bullet flight.

#### speedengineer

##### Well-Known Member
There are a couple common methods used, with varying accuracy.
• Rifleman's Rule. Seems more common, but it not very accurate. Shooter gets MOA values from their 0-inclination range card for the horizontal distance range.
• Improved Rifleman's Rule. What Dog Rocket explained. This method is surprisingly accurate, and imo is easier to do. Shooter multiplies the drop MOA values from their 0-inclination range card by the cosine(shooting angle).
-For both cases, use the wind values for the line-of-sight range with no correction factor
-If you want to convert your hold MOA to inches of drop, you have to use the horizontal distance to do so.

I went through this whole analysis last week as I became curious after some discussion in another thread. I was not surprised at how inaccurate the standard Rifleman's Rule is. I was surprised by how good of a job the Improved Rifleman's Rule did! Plenty accurate for most hunting situations! That said, it's not much more work to type a couple numbers into your cell phone's ballistic calculator.

Plots display the vertical difference for each method relative to the 'actual' ballistic solution as reported by JBM web calculator. Aka, how much you would miss by!
7mmRM was used for this calculation.

Standard Rifleman's Rule is good to 500 yards, or longer if the inclination angle is less than 10 degrees.

Improved Rifleman's Rule is surprisingly accurate for all but the largest inclination angles. FYI, this rule works better with a short zero distance.

And, if you apply no correction for shooting angle at all....

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#### brcfo_outdoors

##### Well-Known Member
This is a pretty interesting video on this exact topic I would suggest checking out.

##### Well-Known Member
the easiest way to get it right is take the degree angle from your RF of it has it and plug it into your Kestrel

if using a COSINE Indicator use it's number and plug it into the quick formula both methods will give you the correct come up on your turret

#### Greyfox

##### Well-Known Member
Back in my early days of “LRH“ at a max of 500-600 yards, I found the cosine formula for slope quite sufficient.

#### 340Wby-4-everything

##### Well-Known Member
If you’re using a kestrel w/AB just use cosine and plug it in. It will do all the hard stuff. If you using something else especially paper dope and you are at 20+ degrees, you can do your manual slope dope correction (cosine x distance) which is better than nothing. I wouldn’t say 600 is far, but as said, it will work fine.
Just to be clear, agreed that for the shooters on this sight 600yds may not be far for a target, but shooting game w/any challenging condition at 600yds (steep angles, heavy wind, etc.), if you are not precise and wound vs. kill an elk, does not only yourself a great deal of dis-service, but shows huge irresponsibility and disrespect for the elk/any game.

#### WiscGunner

##### Well-Known Member
I would suggest the BallisticsARC app from Geoballistics. Simple point your phone toward the target and press the angle symbol and the data sheet will update for the shot. Super easy.

My caribou hunt ladt year was very similar where the best target engagements where from a rock outcropping. Simply check the angles to the different clearing before hand so I could be fast when the time came.

You can even use the map feature and competition mode to predope different shooting lanes simultaneously.

#### msmith57

##### Well-Known Member
OMG, doesn't anyone know how to shoot anymore without all this rule, adjustable scope, range finder, wind speed indicator & computer crap????????

#### Tikkashooter

##### Well-Known Member
its math, using cosines is about as close to perfect ranging as it gets.

#### Tikkashooter

##### Well-Known Member
OMG, doesn't anyone know how to shoot anymore without all this rule, adjustable scope, range finder, wind speed indicator & computer crap????????
If your the Kentucky windage guessing guy and think rangefinders, cosines and adjustable scopes are stupid, why are you even on this page

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