Cosine indicator question

[mattj]

I've seen an exercise like this before, but just for fun, plugged some numbers into Loadbase and did some math.


================================================== =========
Roughly approximating a .300 RUM: BC .711 (240gr SMK), MV 2900fps ---

1000 yard level shot = 25.2 MOA drop

For shot at 20 degree angle:

Rifleman method: 1000 yards * cos(20) = 940 yards; Drop for level shot @ 940 yards = 22.9 MOA drop

Improved Rifleman method: 25.2 MOA * cos(20) = 23.7 MOA drop

Entering 20 shooting incline @ 1000 yards directly into LoadBase = 23.5 MOA drop


"Error" of Rifleman method vs. LoadBase direct incline entry = 2.6% (~6" @ 1000 yards)

"Error" of Improved Rifleman method vs. LoadBase direct incline entry = .9% (~2" @ 1000 yards)
================================================== =========

Of course, the 240gr SMK is a nice slippery bullet, and the projectile is still going almost 1700 fps @ 1000.


The RUM is pretty forgiving at "only" 1000 yards though... let's try a .308 Win:


================================================== =========
Roughly approximating a .308 Win: BC .505 (175gr SMK), MV 2700fps ---

1000 yard level shot = 37.1 MOA drop

For shot at 20 degree angle:

Rifleman method: 1000 yards * cos(20) = 940 yards; Drop for level shot @ 940 yards = 33.1 MOA drop

Improved Rifleman method: 37.1 MOA * cos(20) = 34.9 MOA drop

Entering 20 shooting incline @ 1000 yards directly into LoadBase = 34.6 MOA drop

"Error" of Rifleman method vs. LoadBase direct incline entry = 4.5% (~15" @ 1000 yards)

"Error" of Improved Rifleman method vs. LoadBase direct incline entry = .9% (~3" @ 1000 yards)
================================================== =========

I'll leave the conclusion drawing to the individual reader, 'cause I'm tired

[jwp475]

Originally Posted by Brown Dog

How do you think Exbal is calculating it??!


I'll eat my hat if it isn't applying the cos of the angle to the bullet's drop.

Yes it applies it all right, but it has been proven and Shawn made a great post awhile back explaining why after a certain distanace that useing the cosine on the drops did not work accurately. Using the cosine to multiply the MOA by does not take into account the parabolic curve of the bullets flght nor does it take into account the additional time of flight. If your distance is not too long then the cosine applied to the MOA works reasonabley well, but at the longer distances you will be off useing this system.
That is why in my first answer to the original post I stated "The Same" as niether method is 100% accurate when compared to a quality ballistics targeting soft ware program.
Call Perry sustems and ask them if you do not believe me.

[jwp475]

Originally Posted by mattj

I've seen an exercise like this before, but just for fun, plugged some numbers into Loadbase and did some math.


================================================== =========
Roughly approximating a .300 RUM: BC .711 (240gr SMK), MV 2900fps ---

1000 yard level shot = 25.2 MOA drop

For shot at 20 degree angle:

Rifleman method: 1000 yards * cos(20) = 940 yards; Drop for level shot @ 940 yards = 22.9 MOA drop

Improved Rifleman method: 25.2 MOA * cos(20) = 23.7 MOA drop

Entering 20 shooting incline @ 1000 yards directly into LoadBase = 23.5 MOA drop


"Error" of Rifleman method vs. LoadBase direct incline entry = 2.6% (~6" @ 1000 yards)

"Error" of Improved Rifleman method vs. LoadBase direct incline entry = .9% (~2" @ 1000 yards)
================================================== =========

Of course, the 240gr SMK is a nice slippery bullet, and the projectile is still going almost 1700 fps @ 1000.


The RUM is pretty forgiving at "only" 1000 yards though... let's try a .308 Win:


================================================== =========
Roughly approximating a .308 Win: BC .505 (175gr SMK), MV 2700fps ---

1000 yard level shot = 37.1 MOA drop

For shot at 20 degree angle:

Rifleman method: 1000 yards * cos(20) = 940 yards; Drop for level shot @ 940 yards = 33.1 MOA drop

Improved Rifleman method: 37.1 MOA * cos(20) = 34.9 MOA drop

Entering 20 shooting incline @ 1000 yards directly into LoadBase = 34.6 MOA drop

"Error" of Rifleman method vs. LoadBase direct incline entry = 4.5% (~15" @ 1000 yards)

"Error" of Improved Rifleman method vs. LoadBase direct incline entry = .9% (~3" @ 1000 yards)
================================================== =========

I'll leave the conclusion drawing to the individual reader, 'cause I'm tired

You are spot on here and the facts are indisputable..

[Brown Dog]

Originally Posted by jwp475

That is why in my first answer to the original post I stated "The Same" as niether method is 100% accurate when compared to a quality ballistics targeting soft ware program.

Well...

....they're expressly not the same.


Suggest you re-read the data in Mattj's post.



In his post's final example 'cos applied to comeup' gives a 3" error against loadbase "direct incline entry".....the other gives a 15" error.



Which of the 'error(s)' do you consider significant?




Given the tone of your post, I suspect you will include " 3" @ 1000yds" in your answer.


That could lead into a long discussion as to how one assesses the accuracy (or lack of) of a piece of ballistic modelling [eg how do you know exbal is 'right'....(rather ruins things if your '100% accurate comparator' is actually slightly flawed and out by X" itself) and that would lead into discussion of what constitutes 'within (sensible) tolerance' for ballistic modelling at a given distance.].



If you have the ability to attribute 'correct' or 'incorrect' to a 3" elevation difference @ 1000yds between 2 methods of prediction ....I strongly suggest that you go into business in ballistics .




I'm sorry to say, I simply don't have the energy (or interest) to debate this further.

[jwp475]

Deleted

[dmgreene]

Sounds as though we're violently agreeing

BD I like that one. Can I have permission to use it with my boss and my wife?

David

[mattj]

This came up on the 'Hide today, and somebody posted a really good link:

exterior ballistics


Pretty much the definitive explanation there -- worth the read.