# Cosine Indicator

Discussion in 'Equipment Discussions' started by Len Backus, Jul 28, 2001.

1. ### Len BackusAdministratorStaff Member

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We know that gravity works only on the vector pointing at the earth's center when shooting a bullet. When taking a shot at extreme, angle up or down, the point of impact will be higher than for a shot taken at the same straight line distance over level ground. Drop can be re-calculated for the adjusted distance (perpendicular to gravity vector) by using the trigonometry function called the cosine of the angle. Lots of you guys out there could explain that better and in far greater detail than I.
What most of us care about, though, is how to quickly determine the needed adjustment in the field.

This device calculates the cosine function by use of a rotating level attached to the side of the riflescope. The readout is "cosine of the angle" rather than just "angle" so all you have to do with the readout is multiply this decimal times the lasered straight line distance to the target along the steep angle. The mounting system for the Cosine Indicator is a Leupold QRW ring. http://www.snipertools.com/aci.htm

In my photo it is attached just behind another neat gadget. This green(?) object is basically a spirit level that tells you when the rifle is canted off to one side or the other. RCBS's ballistics software can compute a figure showing how far off to the side and downward will be your point of impact at long range if you cant the rifle at a different angle than that you used in establishing your drop table in the field. My software copy isn't working right now but you would be depressed to know how significant this can be --- hunched up sideways on a hillside somewhere trying to level your rifle from some awkward position.

Darryl: Thank you, of course I meant to say in my post above that the point of impact is higher when shooting at an angle and the point of aim should be lower. It is now corrected above.

[ 08-02-2001: Message edited by: Len Backus ]

2. ### rufousWell-Known Member

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[ 08-02-2001: Message edited by: rufous ]

3. ### rufousWell-Known Member

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That cosine indicator looks really interesting. A couple months ago I was wondering how a guy would go about accurately determining the distance to target that would affect drop (horizontal distance as opposed to line of sight distance) and here is the answer. Cool. Have you actually used one? Is it as simple as it sounds? I have no real feel for how steep a 15, 20, or 30 degree slope is. I have heard some say that a 30 degree slope is extremely steep and wonder how often one would really need to employ such a correction. Obviously the longer the distance and the steeper the angle the more important it would be to make the correction. Thanks for informing us of this useful device. Rufous.

4. ### Len BackusAdministratorStaff Member

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Rufous

I am going on a sheep hunt next week that will give me my first chance to try the unit. I don't have steep hills where I hunt at home. I'll report when I return.

There is one other gadget that works. Basically an upside down protractor with a string or short piece of wire dangling from the center of the base. You hold the protractor in your hand, upside down, and sight it along the angle of fire, reading the angle/decimal indicated by the string.

5. ### kidcoltoutlawWell-Known Member

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Angle Cosine Indicator did it work,thanks,keith

6. ### chris matthewsWell-Known Member

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Badger makes a neat little mount for the CSI that attaches to the rail instead of the scope. Keeps any stress from "accidents" off the scope tube and keeps you from buying another set of rings to get the third one.

7. ### BrentWell-Known Member

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Chris,

Have you got a pic of it, or a link to it? I've never seen one advertised.

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9. ### chris matthewsWell-Known Member

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Thanks for the link Keith, should be on the Badger site also.

10. ### GuestGuest

Angle Cosine Indicator
82nd Airborn Division

"During a recent deployment to Afghanistan, my unit and I had the opportunity to use the Angle Cosine Indicator during training and combat operations. The ACI proved itself invaluable in the mountainous terrain, which we operated in, and was head and shoulders above other angle indicators we used in the past for ease of use, durability, and accuracy. I have recommended the ACI to friends who are going into harm's way, because I know it won't let them down."

"SSG, 82nd Airborne Division

11. ### GuestGuest

I didn't like it on my scope so I bought a Red Dot scope and mounted it on the side. I carry it with the rest of my optics and just use it a seperate device.

12. ### GuestGuest

SPECIFICS. The ACI is an excellent tool for the tactical shooter. It provides the shooter with the cosine, which he can multiply against the estimated range to target in order to arrive at the "gravity" range, which in turn is indexed or otherwise compensated for.

- CW3 - 1st SF, Ft. Lewis, Washington

13. ### BrentWell-Known Member

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Looking at this a while back, I compared the drop at various ranges and angles and found that using the cosine to reduce range didn't match the drop all my programs predicted for shooting at that angle.

I don't have an example on hand but in other words, a shot at 1000 yards at 30 degrees indicated xxx amount of drop, but when compared to the range the cosine said I should use to determine drop, it was very, very different. So, now I'm wondering which one is the more accurate prediction, as I've not done much in the way of angle shooting to verify either one at this point. My palm program works with angles and I'm wondering if it's predictions are BS now.

I did shoot at a bear a while back and the angle was about 25 degrees and 450 yds if I remember right, the program seemed very accurate, although I missed the bear on the long shot but got him on a short one an hour later.

Wondering why I missed the bear, with the same scope settings, I fired and did hit a 6-8" rock 8-10 times in a row afterward right next to where I shot the bear.

If Badger has the mount on their site, I can't find it. I'll call and order one.

I'd like to get some feedback on which one I'm going to have the best results with, either the angle or the cosine... I know one outfit offers both. I do like Badgers mount best tho.

Here's one place that has the angle type indicator. http://www.horusvision.com/hv.cfm?pg=cosine&sku=ACI
The other outfit that had both angle and cosine types, and sold them without the ring was http://www.trgt.com and they've changed hands and now I don't see the product listed...

[ 11-11-2003: Message edited by: Brent ]

14. ### GuestGuest

Hey Brent... Glad you’re interested in the product. I manufacture the ACI and did manufacture "on special order" Angles indicated for Horus Vision. The cosine version does both as the cosines are in five degree increments. This is the method of choice by all of the military Snipers utilizing it. It has been in use, in Afghanistan, Iraq as well as other places on this Globe of ours for over three years. But knowing the "Angle" supposedly works the best with software when shooting at or beyond 1200 yards and at or above 40 degrees. This is because the ballistic coefficient of your bullet changes in flight and the tables in the software packages such as the ATRAG 1&2P, utilize these calcs. However, in order for the math to work with the software, “math is math, science is science”, you must have all of your variables in check. Torque settings, scope height above bore, MET Data etc… The ACI hasn’t failed anyone that I know of in the field, which may be why they have endorsed it on my web site.
Sniper Tools Web Site

[ 11-17-2003: Message edited by: W ]

[ 11-17-2003: Message edited by: W ]