Improved Angle Cosine Indicator

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by Len Backus, Oct 19, 2004.

  1. Len Backus

    Len Backus Administrator Staff Member

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    Ward emailed me notice of this improved version. Wow, that is so much easier to read. I have to buy me one. [​IMG]
    Improved
     
  2. Jeff In TX

    Jeff In TX Well-Known Member

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    Len,

    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 ]
     

  3. 700

    700 Well-Known Member

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    Len, Jeff

    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.

    Rgds

    700
     
  4. Guest

    Guest Guest

    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?

    [ 10-20-2004: Message edited by: W ]
     
  5. Jeff In TX

    Jeff In TX Well-Known Member

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    Len, 700 & W,

    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!
     
  6. Mikecr

    Mikecr Well-Known Member

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    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.
     
  7. Hunter66654

    Hunter66654 Well-Known Member

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    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 ?
     
  8. Guest

    Guest Guest

    Gentlemen: It appears that we are all focused, if not hyper-focused on this issue as well as obtaining the same level if not an extreme level of accuracy. When I originally designed the ACI it was for Military Snipers. Military Snipers are trained to round up in five degree increments and were using protractors as their means of measurements. This is a far cry from the ACI. In any regards, as you know, the cosine numbers are in five degree increments so if the shooter desires to utilize angles, he/she can count up. Although, the cosine numbers are in place to eliminate the step of going to your data book to obtain the cosine number (SOP).

    Commingle the ACI with good ballistic targeting software such as Exbal, or Sierra’s infinity and 1st shot hits on target, and at distance is a reality. There are military personnel around the Globe that are making 1st shot hits at 1700 yards utilizing the .338 Lapua, good glass and an ACI.
     
  9. Froggy

    Froggy Active Member

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    Jeff, I have Snipertools ACI's on both of my competition rifles, one, an Accuracy International 308, the other a tactical 6.5-284 in a A5 Mcmillan stk with all the bells and whistles.

    I bet you $100 that I can hit a man sized target at 800 yrds center mass at any angle up hill or down hill using SniperTools ACI with either of my rifles, you pick the hill and the range and see if I'm bull shiting.

    I hope your body armor works really well [​IMG] [​IMG] [​IMG]

    I've pulled off cold bore shots at at that range many times using ex ball and the ACI, And I can guarantee you this, if the wind is not howling, the elevation of the shot will be dead nut every time. [​IMG]

    Just wondering, do you own a ACI ????? I have placed over 200 center of mass shots at that range with mine [​IMG]
     
  10. Mikecr

    Mikecr Well-Known Member

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    The ACI used to be offered in degrees. I believe it was discontinued in favor of cosine increments(no software needed). Ward was kind enough to sell me a couple ACI's with the older degree windows installed when requested(10deg incr). I also have 2 others in cosine increments(5deg incr).
    I really like the darn things, great overall design. I have many kills using it as part of my system -routine.
    I just wish It had alittle better resolution, because sighters are not an option in real world shooting at angles. Right?
    I get one shot. If I miss, lucky prey lives another day. Thats my rule. [​IMG]

    [ 10-20-2004: Message edited by: Mikecr ]
     
  11. Bob S.

    Bob S. Well-Known Member

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    Is there a rule of thumb that can be applied and get good results? I was going to ask this very thing today but this seems to fit right in here. How does the ACI work if shooting uphill? I was wondering how much stuff you can tape to the stock or in the scope cover. I imagine it would be tough to find a place to shoot at very many angles to get actual numbers. I like the KISS rule for most things so a thumb rule is my preference. I am not shooting for competition though so close would be ok.
     
  12. Mikecr

    Mikecr Well-Known Member

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    Cowboy
    If your not wanting to split hairs at really, really long ranges, the ACI in it's current form will work great for you. You can follow the link to the ACI site and read about it's use. Basically you use it to caclulate a corrected range for the angled shot. Then apply that corrected range when looking up your drop.
     
  13. Jeff In TX

    Jeff In TX Well-Known Member

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    Froggy,

    I do not doubt your abilities behind the trigger, nor do I doubt my abilities behind the trigger. A lot of it comes down to how much training and work you put into this sport

    I spoke in terms of general statistics. Yes, trained professional who practice these disciplines can often make these type shots, excluding shots in strong winds. But in general most folks cannot make a first shot hit on a body sized target at 700 yards or greater.

    I use my mil-dot master with a small lead fishing weight tied to a piece of fly fishing line for my angle indicator. I have one of the original mil-dot masters that the late Bruce Robinson gave me many years ago right after he came out with it. I took a sharpie and added additional degree lines which I fit my needs and requirements. I then put a piece of transparent tape over the sharpie marks so I would not wear them off over time. I also add a ballistic card to the back of the Mil-Dot master that matches my typical location and conditions I'll be shooting at. My system is simple and works for me.

    If you ever get down Texas way let me know. Maybe we can meet and shoot for an afternoon and enjoy a good steak afterwards.

    Good luck and good shooting! [​IMG]

    [ 10-20-2004: Message edited by: Jeff In TX ]
     
  14. Len Backus

    Len Backus Administrator Staff Member

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    Guys, you are right. Ward concedes this fact and even has a very detailed explanation on his website of the reasons why there are limitations to the cosine method af adjustments.

    If you want a method that works in ALL conditions don't use the cosine method. However, this simple method has a valid use for many of us long rangers. In fact, I bet the range/angle conditions present for 75% of our members would be handled accurately all the time. Only when you get to a certain combo of longer range and steeper angle does this simplest method lose practical accuracy. I plan to be proficient in both methods and use whichever one makes most sense in the terrain I plan to hunt.

    The cosine method is plenty good with my 7mm Dakota and .750 BC bullet, for example, at a combo of 600yds and 15 degrees. I have hunted in Montana, New Mexico, Old Mexico, Alaska, Kansas, Wyoming (next month) and Colorado. I don't remember ever having had a shot opportunity steeper than about 10 degrees. The steepest terrain I hunted in was the Wrangell Mountains of Alaska where I expected a steeper shot at a Dall's sheep but never got a shot at all.

    On my next hunt...this weekend in the Missouri River Breaks region of Montana...I will keep track of the angles I encounter with my Leica Laser Locator Plus. I doubt I'll have an animal out there at more than 15 degrees. At 600 yards there just is no significant difference in the methods used.

    In order to force myself to learn the more accurate method while on this trip I am leaving my ACI home and will have an adjustment chart along instead...using the "Improved Rifleman's Rule" as outlined in the William T McDonald article on Ward's website. Then, while on the hunt I will pay more attention to what the angles may reallistically be in that terrain and decide at the end of the trip whether my own future hunting conditions will likely ever benefit from the IRR method. If not I am going to buy one of Ward's new and improved models.

    [ 10-20-2004: Message edited by: Len Backus ]