Cosine Indicator

[Len Backus]

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 ]

[rufous]

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

[rufous]

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.

[Len Backus]

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.

[kidcoltoutlaw]

Angle Cosine Indicator did it work,thanks,keith

[chris matthews]

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.

[Brent]

Chris,

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