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# Cosine Indicator

#43
12-14-2004, 12:35 PM
 Platinum Member Join Date: Jun 2001 Location: Palmer, Alaska Posts: 2,539
Re: Cosine Indicator

With the example I wrote previously:

<font color="red"> 10.63 MOA - 2.954 MOA = 7.676 MOA (corrected MOA you'd now dial) My RSI Ballistics Lab program for example predicts the corrected drop to be 7.68 MOA too.
</font>

Now instead, use the method that takes the bullet path MOA and multiply it by the cosine of the incline angle.

What you get is 8.7 MOA not 7.68 MOA... which is a 7.8" deviation at 770 yards... Now remember, none of our rifles shoot one hole groups at 770 yards, so factor in the additional error due to the rifle and your inability to shoot in one hole at that range and you have the true picture. Did you miss? At 1200 yards it would be MUCH further off than that, using this method it would... even further off multiplying the range by the cosine for a "corrected range"... this is just a fact.

The steeper the angle, and/or farther the distance, the more error these two methods introduce.

How precise do you need to be? Or really, at what point do you find out the method you've been using has failed you as a result of their inherant inaccuracy.

BC,

Your calculation <font color="red"> 1.0 minus 30 degree cosine of .87 = .13 X actual drop of 5.73 MOA = .745 subtracted from 2.9 MOA bullet path = 2.2 MOA </font> is correctly done.

Sorry I wasn't more clear.

Hunter,

The angle should be in reference to the bore line, and set to zero when leveled... has nothing to do with some angle the scope tube happens to be setting at, 20 MOA base or not. The asymmetrical force of gravity changes trajectory when we change the bore angle from the level fire position, which is 90 degrees to this force of gravity.

The 20 MOA base only allows you to elevate the bore line more while maintaining the same POA, or thus compensate for more bullet drop etc.
__________________
Brent Moffitt
#44
12-14-2004, 03:56 PM
 Posts: n/a
Re: Cosine Indicator

Brent: I agree with you; the software programs are a bit more accurate. At least they are with Exbal.
Also, not to confuse the matter, however when the sun light is behind you, your shot will hit approx 1/4 moa low. To the right and it will impact approx 1/4 moa left and left, 1/4 moa right. Heavy overcast? Your bullet will impact 1/2 moa low. A medium (4-6 mph) tail wind? Bullet impact will be 1/2 moa low. Some of the finer attributes of Precision Shooting.

As far as how to mount the ACI... The bore of the barrel must be level. Once the ACI is mouinted, loosen the screw on the side an rotate the body as needed, until the 0 cosine lines up with the index mark. To level the bore of the barrel, pull your bolt out, place a bubble level on the inside rail (that the lugs ride on) until it is level. Then make the adjustment.
#45
12-15-2004, 05:13 PM
 Silver Member Join Date: Nov 2004 Location: GF Montana Posts: 467
Re: Cosine Indicator

ricka's simple COS example
I posted my EARL a couple days ago but some 2nd ammendment lover [img]/ubbthreads/images/graemlins/blush.gif[/img] 1st amm hater [img]/ubbthreads/images/graemlins/frown.gif[/img] nuked it.
As W says:
[ QUOTE ]
shooting and the difference between MOA and inches of drop; they do not work the same

[/ QUOTE ]
MOA = Drop in inches/yds*100 - (1.04" would be ridiculous for the field)
So they are directly related.

Do any of the ballistics packages account for uphill vs. downhill? Shooting my 50 BMG 40 degrees up or down at a target 1300 yds away makes a difference.

I was poking fun of the company for the way they mis-spelt COS as cosign. Not sure what value a useless dictionary def gives other than SPAM. It's not what we mathematicians use.
#46
12-15-2004, 05:40 PM
 Posts: n/a
Re: Cosine Indicator

B1g_b0re: It is not simple to explain this principle and would take me almost two, maybe three pages to do so, and then it would still be difficult for most people to grasp. So… The bottom line is; NEVER use hold-over's in inches of drop when shooting on angles. If you want to hold-over, ONLY use Mil Radians or Minute of angle.

-W
#47
12-15-2004, 06:02 PM
 Silver Member Join Date: Nov 2004 Location: GF Montana Posts: 467
Re: Cosine Indicator

Are you trying to tell a mathematican that 1 MOA is not equal to 1.04" at 100 yards? That angles can't be resolved by vertical and horizontal components?
I think you might be confusing bore line with scope line - the only reason we need the more complicated formula.
I never use hold overs, I have 20 MOA precant on most of my long range guns &amp; 40 MOA on my nightforce equiped 50 BMG's. I simply figure out the drop in inches and calculate the MOA. It works, what should I do, calculate in 1/8 MOA ticks (on my nightForce)?
Now I'm convinced that I need to upgrade to the 2 variable COS formula and aquire ACI.
The 82nd kicks butt - if it works for them, ... well, they are better trained than me - it might work for me [img]/ubbthreads/images/graemlins/cool.gif[/img]
#48
12-15-2004, 06:33 PM
 Platinum Member Join Date: May 2001 Posts: 2,369
Re: Cosine Indicator

You guys holding up okay??

These angle discussions get long sometimes [img]/ubbthreads/images/graemlins/smile.gif[/img] and I'm a little suprised I missed out on this one... should have been looking around a little more I guess.

Nice deer up above, the guy looks a little like Gort from The Day the Earth Stood Still with that blacked out face.
#49
12-15-2004, 06:48 PM
 Posts: n/a
Re: Cosine Indicator

b1g_b0re:
[ QUOTE ]
Are you trying to tell a mathematican that 1 MOA is not equal to 1.04" at 100 yards? That angles can't be resolved by vertical and horizontal components?

[/ QUOTE ]

In a way (excluding the fact that 1 moa = 1.047" at 100 yards) I geuss that's exactly what I have been trying to do only in regards to shooting on angles. However, I think that for some reason we are not conecting. Perhpas you would like to give it a try? Since you are a mathematician, perhaps you can prove to me that a measurement in linear inches of drop is equal and substantive to an angular measurement such as mil-radian or minute of angle when shooting on angles.

-W

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