Cosine Indicator

[Brent]

W,
Thanks.

Cosine will work for both, that part didn't click until you said it.

Here's a problem maybe you guys can help me understand better.

Here's what I'm shooting:
308 Win
175gr SMK
BC .505
2700 FPS
scope height 2.0"
100 yard zero
Standard sea level conditions

Problem: 500 yard target at a 30 degree incline

.87 cosine (30 deg) of 500 yards = 435 yards.

0 degree inclination angle:
Path at 435 yards = -38.09"

Okay, so do we set the scope like we're shooting the 435 yards on flat ground, or do we dial in what most ballistic programs indicate when a 30 degree incline at 500 yards is entered into them like this below???

30 degree incline:
Path at 500 yards = -45.44"

The ballistic programs indicate POI will be -38.10" at 470 yards with the 30 deg incline???

A quick initial test would be to shoot at 500 yards with the rifle still zeroed perfectly at 100 yards, measure the drop, then shoot at 30 degree using the cosine method, and then the angle method and see which one is 8" or so off the predicted path....

Same here for a 1000 yard target:

.87 cosine (30 deg) of 1000 yards = 870 yards.

0 degree inclination angle:
Path at 870 yards = -249.04"

On the other hand the program says...

30 degree incline:
Path at 1000 yards = -313.09"

The ballistic programs indicate POI will be -250.55" at 925 yards with the 30 deg incline???

Once the rifle is well zeroed at 1000 yards, and using an 8' sheet of plywood this one here should be real easy to determine as well. With over 32" of difference between the two methods, it's well worth finding the answer to.

I always figured the programs just ran the cosine, same as we do with the calculator, it definitely doesn't appear that way though.

[ 11-17-2003: Message edited by: Brent ]

 

[700]

The Cosine rule for inclined shots is just a basic "rule of thumb" which provides some degree of accuracy.

The beauty of the Cosine indicator is that if provides a quick and convenient way to measure angle before consulting your drop tables.

A ball version which shows inclination and cant would be fantastic.

Rgds

700

 

[Dave King]

Ohhhh look a puzzle... I like puzzles.

Brent... I'm about to leave for work and will see if I can get time to look this over. I've spend a lot of time on this angle thing and maybe we can both get better at this.

Have you given consideration the height of the sighting device above the bore of the rifle? This true solution to this little problem/portion of the puzzle still eludes me at times.

A'll be baack!!!

 

[Len Backus]

Dave

I thought so!

Someone told me you drove straight through the night to get to California, registered falsely as a voter and cast your vote for AHNOLD. Is this really true or is it just a rumor?

 

[Guest]

Brent... Let me see if I can shine some light on what's really going on when you shoot on an incline or decline.

When you zero your rifle, (lets say that you zero it at 100 yards and you are shooting flat). Because you are shooting "flat", (meaning not on an incline or decline), you have the full effects or force of Gravity pushing downward on the bullet. So when you zero, you have adjusted the sight height above the bore, so the bullet leaves the barrel, arcs up into the full force of gravity and then drops down onto the bulls-eye.

When you shoot at an incline or decline (Angled Shooting) the effects or force of gravity is less; but you still have the same sight height above the rifles bore.
This means that the bullet is leaving the rifle's bore, theoretically but not specifically on the same arc. This is why the bullet hits high; and a knowledgeable shooter will correct for the "gravity" distance to target. The Cosine method of calculation works very, very well for small arms fire and is the method taught in most if not all military and OGA's precision shooting classes.

The cosine method of calculating will give you the bottom leg of the triangle. (Please read the article on my web site.)

To use the Angle Cosine Indicator is relatively simple and works like this:

1) Acquire the straight line distance to target.
2) Aim at the target and acquire the cosine number of the angle that you are holding at by looking off to the side of your scope at the Angle Cosine Indicator.
3) Multiply the cosine number to the previously acquired straight line distance to target. (.7 x 500 yards = 350 yards) or input the angle into your software.
4) Look at your data card for your hold or moa adjustment.
5) Make your adjustment and fire on the target.

As you can see, it is pretty simple.

-W

Sniper Tools Design Company

[ 11-18-2003: Message edited by: W ]

 

[Brent]

W,

I have a way with words most people don't, that is I can generally confuse the hell out of people, where most people don't.

Thanks for describing how it all works, and why.

I have understood what you explained for some time and have thought both methods were one of the same, but as I'm trying to point out, they are NOT the same. I can't understand why the ballistic programs are so far off when simply entering in the incline angle, verses using the cosine and simply using the flat land, or horizontal range.

In the post above, I attempted to show the cosine method's prediction, and the drop in inches first. (.87 cosine of 500 yds = 435 yards (-38.09")... Here, you look at the flat land drop chart and hold for a 435 yd shot when the actual range is 500 yards with the laser. The "important" part to note here, is that it's telling you to compensate for 38.09" of bullet drop at 500 yards when it's on a 30 degree incline or decline.

W,
You are saying this is the correct way and have found it accurate. This is also the most common way I've seen it figured too, and you explained why very well I might add.

We know .87 is the cosine for 30 degrees, so we look at what the "Palm program" predicts using this "30 degree" incline, ATrag, Exbal, which ever program you happen to use.
-45.44" is what is predicted using this method, "NOT" the -38.09" the other method indicates, hence the problem. Over 8" of difference at only 500 yards, and at 1000 yards it is over 32" of difference between the two methods of prediction.

I hope this clarifies what I was trying to say a little bit more.

Dave,
The scope height is above with the load too, but it's 2.0" above the bore in case you missed it.

I'm using Exbal and ATrag on the PDA, so it would be nice to know the angle function in them work, or don't work.

I'd simply multiply the laser range by the cosine and then enter the flat land range it predicts if it is the accurate one to use. If the incline angle function on the PDA programs proves they are what is accurate, I'll just look at the cosine and determine what angle it represents and use it with the program. Either way, I've got to find out which method is less accurate.

If it turns out, my impacts are between the two somewhere, then I'll be scratchin my head over this....

If the angle function on the ballistic programs are wrong, they will be worthless to anyone using them, as all mine predict nearly the same thing as one another, and they are not even close to what the cosine calculation predicts.

Interested in your thoughts on this too, Dave. Anyone else too?

[ 11-18-2003: Message edited by: Brent ]

 

[chris matthews]

Haven't had much luck with any ballistic program- I generally treat them as a guide and not as the absolute. They are meant to get you close, then you fine tune from there.
I would go with W on this, Marty (Badger Ord.) and I have discussed this thing up, down and sideways. I have one on my rifle with the very first Badger CSI mount and the bottom line is it works. Real world firing from Missouri, Texas to South Dakota.
That's why military snipers keep log books of their rifle with real world shooting, not relying upon ballistics programs.
I am not saying they are bad but are meant as a guide, nothing more. Some are more accurate than others. Some are not.

 

[Guest]

Here is a link that you might be interested in reviewing. Snipers Hide Forum - Angle Cosine Indicator

 

[Brent]

After reading throught the topic, which I missed, as I don't post there all that often, I can't hardly believe the resolution isn't 1 degree with the ACI. It is an analog device that can easily be broken down visually with no trouble at all. I can easily break down the .001" marks on my dial caliper to .0002" increments, and my 2 MOA tics on the R2 reticle the same way. How in the hell would anyone use a mildot reticle without this practice also.

I wonder if the guys using ATrag1p on the Handspring Visor were doing the manual cosine calcs, or just entering the angle into the "Angle" function on the ATrag program.

I'm guessing that the drum that the cosine numbers are printed on is all that is needed for the left hand version?

Is the drum not symetrical and the ACI cannot be disassembled and the the drum reversed to use on a lefty? Or can it be reversed? My dad shoots lefty.

 

[Guest]

The wheel can be reversed, however it is better to utilize a wheel that is made for a lefty because of the interior design. If I have advance notice, I can manufacture "special order", a left handed version.

 

[Birth Controller2]

Brent,

I have seen the same discrepancies between ballistic calculators and the results from using the Cosine multiplier.

Wish I knew why they're so far off. You wouldn't think that all those professional snipers were wrong in their praise of the ACI?

BC

 

[danacobb]

I'm definately no expert on the subject and haven't done much angle shooting in the real world but I have read quite a bit about the subject. I've read the difference between the angle cosine difference and some ballistics software is that the software takes into account for the time of flight. If you figure out your angle and take the cosine of it; that'll give you the distance that gravity has and effect on the bullet (ie. the horizontal distance). This does not however take into account that the bullet is still traveling the ranged distance to the target (actual distance). I know Exbal and the Sierra software take these factors into account. I would think a combination of the ACI to get your angle and then using the angle chart to get the scope adjustment would be the correct way. But like I said, I have no real world experience in this. Interested in your findings; keep us posted.

[ 12-06-2004: Message edited by: Cobber ]

 

[Birth Controller2]

Interesting. That makes some sense, but it assumes that the flight time would be the same.

Wouldn't the bullet get there sooner when flying at an angle to the target for the same reason the yardage is shortened, i.e. less gravity effect?

TBC

I'm definately no expert on the subject and haven't done much angle shooting in the real world but I have read quite a bit about the subject. I've read the difference between the angle cosine difference and some ballistics software is that the software takes into account for the time of flight. If you figure out your angle and take the cosine of it; that'll give you the distance that gravity has and effect on the bullet (ie. the horizontal distance). This does not however take into account that the bullet is still traveling the ranged distance to the target (actual distance). I know Exbal and the Sierra software take these factors into account. I would think a combination of the ACI to get your angle and then using the angle chart to get the scope adjustment would be the correct way. But like I said, I have no real world experience in this. Interested in your findings; keep us posted.

 

[Birth Controller2]

Oh and BTW ...

A Leupold Rifleman scope ring works good for mounting a ACI. They are sectioned vertically.