## Angle Cosine Indicator (ADI) Aids Hunters

Unlike benchrest shooters, hunters need to be able to make shots with significant up-angles or down-angles. That whitetail buck may be poised on a ridgetop above you, or in a valley below. When making an angled shot, the hunter faces a complex ballistic solution. This is because the angle of the shot alters the effective ballistic distance to the target. Whether you shoot up-angle or down-angle, you must adjust your elevation “clicks” as if you are shooting a shorter distance. See the diagram below. The drop of your bullet is a function of gravity, which remains constant. When you shoot at a steep angle, the actual bullet travel over the ground will be less than the sloped distance to your target.

But how do you determine the flat-line or “gravity-corrected distance”? There’s a simple tool that will do the job: the patented Angle Cosine Indicator (ACI®) invented by U.S. Army Veteran Ward Brien.

When you aim your rifle at an angle, the ACI shows the cosine value of your intended shot by means of a highly visible index mark. You simply multiply the true, sloped distance to your target by the cosine value (as a percentage), to get the corrected, flat line distance to target, i.e. the bottom leg of the triangle. Then set your scope’s elevation accordingly. For example, if you range the line of sight distance to your target at 400 yards, and the ACI shows a cosine value of 0.87 (for 30 degrees), then your flat-line “gravity-corrected distance” is 400 x 0.87 = 348 yards. Now Dial your elevation for 350 yards (from your come-up table).

This simple multiplication method works well for typical 100-300 yard hunting distances, but it’s not perfect. For longer-range shots, out to 1000 yards, some other factors come into play. The **most accurate method for long ranges is to input the cosine number into ballistic software**, such as Exbal Ballistic Targeting Software, that runs on a PDA or smart phone. The software takes into account the fact that, during an angled shot, the bullet must still travel the full distance to target, and will have a longer time of flight than when covering the flat line distance. At very long ranges there can be as much as eight (8) MOA difference between the simple multiplication method and the solution generated by the ballistic software. NOTE: ACI Inventor Ward Brien has posted a Comment to this article which explains in greater detail why inputting the ACI value into a ballistics program is the “preferred method”.

The Angle Cosine Indicator costs $145.95 from Sniper Tools. The ACI is made from aircraft grade aluminum, anodized flat black. Angles are laser-engraved onto the body in five (5) degree increments. The lens is water-resistant, shatter-proof, and shock-proof. Completely mechanical, there are no batteries or electronics to fail. For more info, visit SniperTools.com, or call (818) 359-0512.

SniperTools.com also offers an Angle Degree Indicator (ADI) for $114.95 (Civilian model). This shows the actual angle from horizontal. If you have a mobile ballistics solver, you can simply input the angle and the ranged yardage and the solver will provide the flatline “gravity-corrected Distance”. Some shooters find it easier to think in terms of the actual angle deflection from horizontal.

There are several ways to mount an ADI or ACI. We prefer a 90° slotted mount attached to a Picatinny rail. SniperTools.com sells a Badger Ordnance ACI/ADI Rail Mount for $60.00.

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Tags: ACI, ADI, Angle Cosine Indicator, Badger Ordnance, hunting, Optics, SniperTools.com, Ward Brien

You may want to add some info to your article about the ACI. When using it at longer distances you need to put the lazered distance as well as the cosine into a software package such as Exbal. You will find that if you just multiply the cosine x lazered distance you will get a different answer than by putting all of the data in Exbal.

This has been proven to be true by shooting at longer ranges using both methods.

Check out this article: http://www.longrangehunting.com/articles/angle-shooting.php

One more thing to add to this article.

Be very careful whey you “ZERO” the unit to your rifle bore. If you tighten the shroud screw too tight, the unit will bind and will not function properly. Unless Waren has not fixed this problem, you can fix it yourself by removing the two teflon washers inside the unit. I’m sure Waren would not like this idea, but it’s the only fix to this problem. Good luck!

There are three different methods of utilizing the Angle Cosine Indicator and one is more accurate the other.

METHOD #1: The original method and least accurate is called the “Rifleman” method. This is where the shooter multiplies the sloped distance to target by the cosine number. Then obtains the corrected for gravity hold from their data card.

Method #2: This is the “Improved Rifleman” method. This is where the shooter multiplies the cosine number directly to the hold as depicted on their data card. As an example, if you are shooting a .300 Win Mag at a distance of 600 meters, (Where I live the elevation is 8000 feet ASL) my hold as depicted on my data card equals 3.6 mils.

If I am aiming at a Big Horn Sheep at 600 meters and I am aiming up on a 45 degree angle, (Cosine .7) I would multiply .7 X 3.6 mils = 2.52 mils. My corrected for gravity hold is now 2.5 mils.

Method #3: This is the most accurate method and is where you would enter the cosine number into the data entry point of the Night Force Ballistic Targeting Software. The reason that this is the most accurate method is because the software takes into account the following:

a. The bullet‘s time of flight is, [for all intents and purposes], the same to the target.

b. The bullet’s own unique ballistic coefficient.

c. The bullet’s own unique velocity.

d. The bullet’s own unique deceleration curve.

e. The commingling of the meteorological data of the area that you are shooting.

Depending on the distance to target and the slope of your aim, method #3 can be up to 8 minute of angle more accurate than Method #1. [Editor’s note: The difference in accuracy is much less significant inside 400 yards, at normal hunting distances.]

One minute of angle equals 1.047 inches per hundred yards. At 800 yards, that would equal 67”.

As far as over tightening the ACI, I strongly suggest reading and following the instructions that are included with each ACI purchased. ONLY tighten the shoulder screw to five inch pounds of pressure.

Scope rings should not be tightened any more than 15 inch pounds of pressure MAX, with 12 inch pounds of pressure recommended.

READ THE INSTRUCTIONS AND FOLLOW THEM.

Best Regards,

Ward W. Brien

President

Sniper Tools Design Company, LLC

Why would I want something like this when laser range finders are readily available with angle finding/compensating capability built right in?

I think this a great tool but i’m cheap so I’ll stick to the mil-dot master I’m not in that big of a hurry.

Because batteries die!