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Cosine Indicator

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Unread 12-13-2004, 01:50 PM
Silver Member
Join Date: Jul 2003
Location: Orofino, TX
Posts: 164
Re: Cosine Indicator

W, either way it was a nice shot and he did use the cosine indicator. David was along side his Son Mike and was calling the shot. I have a photo of it if I knew how to post it. I'll send it to you if you want it and /or want to post it.
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Unread 12-13-2004, 07:24 PM
Posts: n/a
Re: Cosine Indicator

hunter66654: Herre is the image that you sent to me to post of Mr. Powers. I am not sure if he is comfortable with his face being shown on the web, so I blacked it out. But here is the image. Very Nice Muley!
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Unread 12-14-2004, 06:52 AM
Bronze Member
Join Date: Jul 2003
Posts: 32
Re: Cosine Indicator


I printed your post out for study. You lost me. [img]/ubbthreads/images/graemlins/wink.gif[/img]

The "Improved Rifleman's Rule" does work (as you mentioned)

It's the "Rifleman's Rule" that is put forward on the ACI site that is sorely inaccurate.

3650 fps
.271 BC
55 grain Sierra BlitzKing (.224")
1.8" scope height
0 mph wind
30 degree angle
250 yd Zero
65 degrees F
30.00" mercury
69% humidity
966' above sea level

Here's what I get using JBM Trajectory Calc:

400 yards path -12.3" 2.9 MOA (1/4 minute)

400 yards path w/30 degree angle -10.6" 2.5 MOA

ACI/using RR (cosine .87 X 400 = 348 yards) -5.6" 1.5 MOA

ACI/using IRR - .87 X 2.9 = 2.5 MOA


<font color="red"> Now, this is an easy adjustment; a simple equation that will put you very close to right on target, if not dead on. However there are two ways to obtain this and one is a little more accurate than the other.

I think what Ward is refering to here as being more accurate is the method that is more comonly used, by Sierra and most every ballistic program I've run accross... including Exbal.

1.0 minus the Cosine of the incline/decline angle multiplied by the actual drop from your boreline (not to be confused with bullet path). Subtract this amount from your level fire come-up.

First - It's easiest to do this math in MOA verses inches then converting to MOA.

770 yard shot at a 35 degree angle.
LOS to target is 770 yards, and corrected horizontal range is meaningless here.
35 deg = .819 Cosine
1 - .819 = .181

"Vertical" drop from boreline = 16.32 MOA at 770 yards - (this remains constant, level fire or incline fire)

"Level fire" bullet Path = 10.63 MOA at 770 yards.

16.32 MOA * .181 = 2.954 MOA

10.63 MOA - 2.954 MOA = 7.676 MOA (corrected MOA you'd now dial) My RSI Ballistics Lab program for example predicts the corrected drop to be 7.68 MOA too.

The easiest thing that I have done to help simplify these calculations in the field was to ask Jim Ristow at RSI ( ) to modify the drop from boreline results in his program to read in MOA, not just inches etc... thanks Jim!.

What you can now see by having the DROP and PATH column results "both" in MOA is the exact MOA "difference" between the two.

The reason this becomes important is that in the field we use our bullet path - not the drop from our bore line, which is what we now find is needed for these calculations to be ACCURATE.

If you compare bore line DROP and bullet PATH MOA, you'll notice that there might be an average of 4.0 MOA difference between the two from 300 yards to 1200 yards, or something like that... maybe it's 3.75, 5.5 MOA or some other number. The important thing is to know "what number" it IS to add to the bullet PATH numbers you have commited to memory already so you know the bore line DROP in an instant also.

This eliminates the need to carry or memorize a bore line DROP chart also... the point of this whole thing, it eliminates a step in the math involved with virtually no loss of accuracy of the firing solution.

If my PATH requires 10 MOA correction at 750 yards, and a 35 degree incline is encountered, I add my "set" 4 MOA to the 10 MOA for 14 MOA, then multiply by .181 for 2.5 MOA to subtract from the original 10 MOA.... so I dial 7.5 MOA.

Pretty simple really, but very ACCURATE.

Another method, much more accurate than the "Slant Range converted to Horizontal Range" method (which our military has embraced), but still lacking for accuracy is where you multiply the bullet PATH MOA correction by the cosine of the incline angle and simply dial this new MOA solution.

Sierra explains this all much better than I can on their site here.

They also explain in detail WHY, and WHAT is going on that effects incline fire.

Hope that helps and I didn't confuse.

Here's another discussion from last year too.;m=107600045

[/ QUOTE ]
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Unread 12-14-2004, 08:15 AM
Join Date: Jun 2007
Location: North Idaho
Posts: 1,877
Re: Cosine Indicator

I also muliply the cosine and the moa comeup. This has given me first rounds hits on my 10" disk at various angles out past 1200 yards with my 338 Edge. That is a nice mule deer Mike shot.
Shawn Carlock
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Unread 12-14-2004, 08:52 AM
Bronze Member
Join Date: Jul 2003
Posts: 32
Re: Cosine Indicator


Using my example above and using your method I came up with something close. Check the following ...

1.0 minus 30 degree cosine of .87 = .13 X actual drop of 5.73 MOA = .745 subtracted from 2.9 MOA bullet path = 2.2 MOA (I got 2.5 MOA above)


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Unread 12-14-2004, 10:19 AM
Posts: n/a
Re: Cosine Indicator

Birthcontroller… There is a reason that there are established standards for using cosine. My advice to you is to start using them. Once you do, you will be like everyone else and you will most probably experience success. Simple is as simple does. [img]/ubbthreads/images/graemlins/wink.gif[/img]
Now, as far as the data that you stated that you are using; there are two problems with it. 1) You are using altitude instead of uncorrected for altitude Barometric Pressure. Altitude should only be used when you do not have the means of utilizing the Barometric pressure. 2) After I ran your data, I came up with totally different elevation adjustments then you did, in fact up to a minute off. I am now beginning to wonder if you know the difference between inches of drop and MOA as they are Not the same. But if you have the time, give me a call. I would like to help you out.
Yards = 400
Vel = 2298 @ 400 yards
Energy = 645
Drop Inches = -28.7
Elv./hold = 3.25 moa
Flight Time = 0.4155 Sec.
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Unread 12-14-2004, 11:57 AM
Silver Member
Join Date: Jul 2003
Location: Orofino, TX
Posts: 164
Re: Cosine Indicator

W, let me throw another question into the mix. My scope is mounted with NightForce rings and a NightForce 2-piece 20 MOA base. Do I want to mount the ACI parallel to the bore of the rifle and what does the 20 MOA base do to the situation ?
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