This device was throughly tested and evaluated on one of the tactical forums web site.

Bottom line and it's something I discovered in some of the long range and tactical courses I've taken. Using a cosine for long range shooting is not as accurate as most folks think. At relativly short ranges and mild angles those devices work well, but at longer ranges and steeper angles they are not very accurate.

Cosine angles will give you the corrected yardage to your target. However, your bullets time of flight is still longer to that cosined target than it would be if it was shot at on a level range.

Sorry I almost confused myself. I used my ballisitc program for these numbers.

Let say I'm shooting at a level range at 500 yards. My bullets time of flight was .653 seconds to the 500 yard target.

Now I'm out west on a hunt and I want to make a shot at a rock chuck. My laser shows it's 650 yards to the rock chuck, but it's at a steep angle. I use my cosine indicator and it shows the actual yardage to the target is 500 yards.

I make my adjustments for a 500 yard shot, pull the trigger, but miss. The reason is my bullet is still traveling 650 yards even though the corrected angle to the rock chuck was only 500 yards. My bullets time of flight was really .850 seconds to cover that adjusted cosine 500 yards. That means my bullet was in flight and being pulled down by gravity almost 25% (23.2%) longer than it would have been if it was fired on a level range.

I hope this makes sense.

[ 10-19-2004: Message edited by: Jeff In TX ]

__________________
Jeff

Mathew 5:16

Distance is not an issue, but the wind will make it interesting!

All the Cosine calculation does is allow for ,in a very general way, the change in gravity acting perpendicular to the Line of Sight.

In Jeffs example, the time of flight is 23.2% longer than for a 500Yard target, but the force of gravity acting along the line of sight is 23% less than normal gravity. It works the same as the way the cosine rule is used to find the wind acting along the line of sighth. Eg, a 10 MPH acting at 1 O' Clock when the target is at 12 o clock, will have the same effect as a 5 MPH wind acting left to right.

5MPH=10MPH * Cosine(60)

Depending on accuracy required, it works very well at short to medium ranges, but just like everything else in long range shooting, small inaccuracies in calculations give large inaccuracies in targeting solutions at long range.

For long range, it is best to play with a ballistics program until you get some angled drop charts that match what you see in the real world.

Guys: Yesterday David Powers of “Clearwater Bows” in Orofino, Idaho phoned me for another order of ACI’s. To make a long story short, he conveyed a story about an Elk hunt that he and his Son did several weeks ago. His Son was shooting a 7mm Mag., (I think with 160 grain Accubond’s) and using an Angle Cosine Indicator, shot and killed an Elk at 1,080 yards while holding on a thirty degree angle. One round and the Elk went down. But he uses the ACI a little differently then the way the military and Federal Agencies train. Instead of multiplying the cosine to the distance to target, he multiplies it to his moa holds from his data card. Interesting… And he swears by it.

Oh and Jeff in Texas... You are mathematically correct in what you are saying; however does this mean that you would feel comfortable standing in front of me at 1000 yards while only utilizing the cosine method for distance correction?

Great open feedback on this topic. I feel this is the most proactice way to get out as much information on a topic as possible so everyone else understands the bigger picture.

Now for W's question?

"Oh and Jeff in Texas... You are mathematically correct in what you are saying; however does this mean that you would feel comfortable standing in front of me at 1000 yards while only utilizing the cosine method for distance correction?"

Answer, I dunno, which one of us is on the recieving end at 1000 yards? I don't use a cosine indicator. I use ballistic tables from my RSI program.

As my tag line states, "Distance is not an issue, but the wind can make it interesting!"

The other added bonus on this, there are very few first round hits on soft targets beyond 700 yards and more so at 1000 yards. add in wind and angles and that percentage goes up even higher. That what sighters are for!

Great topic and responses....thanks!

__________________
Jeff

Mathew 5:16

Distance is not an issue, but the wind will make it interesting!

I use the ACI for angle in DEGREES.
Exbal and Sierra both utilize this angle(in degrees)correctly.
Sierra's site has an explanation of angle methods, which is damn good.
Each increment on the ACI =5degrees.

The best improvement w/regard to precision use of an ACI, would be a better approach in it's design and use. Needed are single degree marks (very fine -like an R2 reticle)on the inner wheel, and a single line etched across the window). With this, you could discern a 13deg shot, for example, and load that into your software. As it stands, I have to estimate tweeners. I can't modify the window, because it's full of numbers. And the new design appears to have a narrower window(unusable for applying to the older style body).

But it could always be said that it's better than nothing.

I second what Mike says. I would love to have an ACI which just shows angles in degrees in 1 degree increments if possible or at least the smallest increments that you can read. I know David powers ( in fact I was the one who found the ACI on the web and told him about it )and his has taught at least 10 of his friend in the mountains of Idaho to use it as he does. If you know the angle you can make a chart for come ups for distances at various angles and tape it to your stock or if you have your ballistic software ( such as Exbal ) in a PDA you can get the come ups from it . What are the chances of getting a ACI that just shows angles ?