Angle Cosine Indicator - ACI

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timsinc

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Not sure if this is the correct thread for something like this but here we go!

I have a Horus Angle Cosine Indicator (ACI), don't really need it as I don't plan on getting into LR shooting anymore.

Do people still use these? any value in it? I would think with modern rangefinders giving true horizontal distances these days these are obsolete but ya never know. It has a little bubble level on the top, slip on cover and attaches to a pic rail. Ive found ACI from 40-400$ on Amazon and US store sites but not much around Canada.

Any info is appreciated, and Happy Holidays!

https://photos.app.goo.gl/ixX5csBDgDA8byxv9
 
No point. Unless you are doing high angle shooting, you dont need one. Never seen one being used, either at a match or on a range.
 
Yeah, not real useful these days. Can be used as a backup analog device to get angle to target (in a high angle situation) if your LRF has died and you have to do the math longhand. It's helpful to actually look at a dope chart and see at what angle and range angle to target actually starts affecting your dope enough to matter.
 
Still useful for someone doing loads of high angle shooting and can actually perform the trig calculations.

I have seen a few strapped to pic rails at the range but the guys that have them mounted have no clue how to use them nor do the math. They are mostly decorations for guys that like to accessorize their toys.
 
The math is easy.

If you are angled down 30 degrees from horizontal, and it is 1000 yards direct to the target on that angle, the actual horizontal distance is:

Horizontal distance = cos(30 degrees) X 1000
Horizontal distance = 0.866 X 1000
Horizontal distance = 866 yards

If you look at the pics of the gauge in the OP, you’ll see that 30 will match up with 0.86, so it’s just doing that little bit of math for you. If you have a phone, you can use the calculator to find cosine of that downward angle.
 
I know of only one guy (of quite a few) that has one on his F Class rifle.
I doubt he has ever made use on it at one our matches. He probably installed in his newbie years -did not use -and just left it on there.

Our targets / backstop is a fair bit higher in elevation in relation to all our shooting mounds.
At SPRA our backstop is a large hill made in the middle of a flat cow pasture.... even at 1,000 yards all of our barrels are quite flat.

Unless you shoot at really extreme angles (on a regular basis), having one of these on your rifle is "bling" and nothing more.
You'd be better off spending those extra funds on your optic or bipod.
 
Last edited:
Munkey1973 said:
…Our targets / backstop is a fair bit higher in elevation in relation to all our shooting mounds.
At SPRA our backstop is a large hill made in the middle of a flat cow pasture.... even at 1,000 yards all of our barrels are quite flat.
Click to expand...
Just out of curiosity, how much higher are the targets compared to your shooting position?

My local range is only 100m, but the targets are quite elevated, and you really notice it. Using the BallisticsARC app, it said my shot angle was 4.3 degrees. The math here fully supports your observations, that it just isn’t going to matter.

cos(4.3 degrees) is 0.997, so if the same angle was used for a 1000 yd target, it would have a horizontal distance of 997 yards. Close enough!
 
Thanks for the info, I kinda figured they were a thing of the past with modern rangefinders and phone apps taking the place of this thing.
 
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