Here is the process I go through to get ready to hunt in the mountains, where inclination angle shots are common. 1,200 yards is the practical limit of the 308 at altitude, so the data stops there. I start with my drop from the bore line in inches and convert it to MOA. Do not confuse this with your come up data, they are vastly different. For my load in the 308, I will use trajectory data at 6000 feet above sea level with the temperature at 45 degrees Fahrenheit and the barometric pressure at 29.6

This trajectory data was collected in the real world, and may or may not agree with your favorite software program. It is an important lesson to learn, that you collect trajectory data in the real world, not just rely on what your computer tells you. You can save time by collecting real world data points every 200 yards and use your computer to fill in the gaps. If you are going to dial dope instead of use a scaled Reticle, I would recommend getting real data points every 50 yards. Time to build some useful tables, starting with "Drop from Bore Line"

Range_______Bore Line Drop 'MOA'_______Bore Line Drop 'Inches'

200______________4.78______________________10

300______________7.64______________________24

400_____________10.98______________________46

500_____________14.33______________________75

600_____________17.99_____________________113

700_____________21.97_____________________161

800_____________26.38_____________________221

900_____________31.31_____________________295

1000____________36.77_____________________385

1100____________42.89_____________________494

1200____________49.67_____________________624



Next we need the angle correction factor which comes from the Sine/Cosine tables.

SHOT______________CORRECTION
ANGLE______________FACTOR

5_________________.004

10_________________.015

15_________________.034

20_________________.060

25_________________.094

30_________________.134

35_________________.181

40_________________.234

45_________________.293

50_________________.357

55_________________.426

60_________________.500

65_________________.577

70_________________.658

75_________________.741

80_________________.826

85_________________.913

You no longer need the bore line drop in inches table, so drop that and copy everything else onto one page, or write it onto a second Pathfinder attached to your scope. Now let's work an example that would be common for a hunter with a 308 in mountainous terrain.



OPPORTUNITY: A monster 5 x 5 mule deer buck presents an 800 yard shot with an inclination angle of 40 degrees. No Worries!


Take your bore line drop in MOA at 800 yards (26.38) and multiply that by your 40 degree correction factor (.234), now subtract this number from your MOA come up at 800 yards.

26.38 X .234 = 6.173 MOA

My come up for 800 yards from my 200 yard zero is 20.5

20.5 - 6.17 = 14.33 MOA

14.33 is exactly how many minutes I will hold my 200 yard zero above to take this 800 yard shot at a 40 degree inclination angle. When trying to kill live targets it is important to do as many calculations ahead of time as possible. This is why I convert my bore line drop to MOA, so I only have one multiplication problem and one subtraction problem to get my actual come up from zero. If only reading the wind were a simple math problem!

Sam,
Unless I understand him wrong, Tiro suggests here
[URL]
http://www.longrangehunting.com/cgi-bin/ultimatebb.cgi?ubb=get_topic&f=3&t=001637
[/URL]
that scope height MOA and drop from bore line is * by cosine?

quote:

---

Blaine, I believe the formula should read:

BP = D*cos(a) - SH*{(R - Rz)/Rz} - Dz*R/Rz

Where:
(a) = bore angle to the horizontal
BP = bullet path
D = drop, horizontal range
SH = sight height
R = range
Rz = zero range
Dz = drop at zero range, horizontal


From this formula we can see that the BP correction at any range is: D*(1 - cos (a))
This correction is always substracted to your bullet path.

Maybe a more practical solution to this problem is to use the scope angle of elevation above the horizontal.

For example a 175 SMK @ 2680 fps, std. cond., needs to compensate for: 2.5" (drop) + 1.7" (sight height) = 4.2" to zero at 100 yds. This is, 4.0 MOA of elevation are already dialed into the scope when you are zeroed at 100. If your come ups from this 100 yds zero to 1000 yds is 36.6 MOA, you really have 40.6 of elevation into the scope.

Think in terms of "scope elevation" instead of drop or comeups!

To correct for any angle at any range multiply "scope elevation"*cos angle

This elegant solution was suggested by BMG Mike back when we had some discussions at SH. It is faster than the solution explained in the Sierra site and produces results with similar accuracy.

For any angle <30º, if your base zero is 100 yds you can just use: come ups * cos angle and you'll be veeery close.

Remember all these methods are a simplification of the problem, because they disregard the small variations of bullet drop due to the air pressure and gravity effects on angled shots (which are taken into account in some programs like the Infinity), but these effects are minor and of opposite signs and almost cancel each other out.

---

It would be interesting to see what answer Tiro gets by figuring the shot example I gave in my last post, considering my SR-25 has a borline to sight line difference of 2.6 inches. It would also be interesting to see how fast he can break a shot using his method in the real world, where targets sometimes present a very small time slice to break the shot.

Truth and Objectivity are Mutually Inclusive

That was just an explanation on how to derive the numbers, NOT a suggestion on a correction method on the field.
Sam's method is correct, but it is faster to use: "scope elevation"*cos angle (one math opearation instead of two). You just have to remember your data in "scope elevation" instead of come ups.

Tiro....Use my 800 yard example, explain what data you carry into the field, and show what the exact calculations are. I am always interested in more speed.

Truth and Objectivity are Mutually Inclusive

Sam,
What is your MV and BC? The BP is 29.60, I wonder what station pressure you are running? One program figures pressure at altitude to be 23.80, the other figures it at 23.62???

Tiro,
Will you run through the 800 yard example of Sam's? I'm not sure I quite understand the sight elevation method you're describing completely. I can't seem to make it work.

Total drop from the bore line simply multiplied by the cosine... is not working as I had thought it did.

I'm looking at multiplying come ups by the cosine to see what it produces again. I had thought the simple drop from boreline * cosine was most accurate and that it was the same as these damn programs predict, I did find this to work the last time I looked at it, it's not working at all now.

There was a thread at SH, "Slant Range Exersize" which is no longer there, but BMG Mike, you and someone else had been discussing the accuracy of various methods, Range * cosine, Drop * cosine, Path * cosine, Come-ups * cosine etc...

I ran the numbers on some of the methods back then, and what I found THEN is NOT what I'm finding now. Total drop * cosine (26.38 MOA * .766) for Sam's load = 20.21 MOA, that isn't even close to right.

Come-ups * cosine (20.5 MOA * .766) = 15.7 MOA, kind of close but isn't right either.


One program I'm using predicts 14 MOA, another 14.36 MOA.

I can't get Tiro's method to work how I'm doing it???

No way to run THAT long ass FORMULA in the field, Tiro.

Brent, the muzzle velocity I believe to be 2660 fps., the B.C. starts about .500 and probably goes lower as the bullet flies because it is a Sierra 175 MK.

Truth and Objectivity are Mutually Inclusive

Sorry for the late reply guys, I have some other things to do too

Sam, how did you calculated your real world "bore line drop"?

For your example:
scope elevation at 200 yds = 2.6" + 10" = 12.6" / (2*1.047) = 6.02 MOA
scope elevation at 800 yds = 2.6" + 221" = 223.6" / (8*1.047) = 26.70 MOA
corrected scope elevation at 800 yds, 40º = 26.7*cos 40º = 20.45 MOA

So, for a 800 yds shot at a 40º angle I dial: 20.45 - 6.02 = 14.43 MOA up from my 200 yds zero.

Tiro....Run a laser through the bore on to the target at 100 yards, and then set the scope parallel to the bore 2.6 inches above the middle of the vertical extremes that the laser can reach on the target. A couple of 2x10 ft. boards with a cardboard target stapled to the bottom will get you to 700 yards. After that we go to a vertical cliff with a smooth rock face, hold on the upper ledge, and drop the 100 ft. tape down to measure.

Truth and Objectivity are Mutually Inclusive

I belive S1 has this one right. I used the same method a couple years ago to make my first LR kill. Shot was 763 yards with a 15 degree up hill angle. Hit was dead center of mass.

In the field I carry the ballistics for my rifle in drop inches from zero along with real drop inches. I also carry the multiplaction figures for the angles in 5 degree increments. I simply multiply my drop from boreline by a given figure and subtract that from my drop from zero. Works every time. I set my scope the same way for my buddie who shot his ram at 600 yards with a 20 degree angle. He collected him a moment later.

Michael

Its not about how many rifle you have, its about how well you can use what you do have.

S1: After reading your post, I found it to be interesting and very in-depth. However, it leads me to ask you where did you learn your methodology? When I go out, I range my target, multiply the cosine number to that distance, look at the data card I made earlier that morning, set-up and send one downrange. Why all the Magilla?

http://www.snipertools.com

W, have you read this article?
http://www.exteriorballistics.com/ebexplained/article1.html

Howdy W, and welcome to the board. The reason I go through the trouble to do it this way is because it gives me a much better answer. I need more precision when shooting past 300 yards in steep terrain. If you are willing, take an example past 800 yards, and figure a solution with your method, and then with the method I use. You will quickly see that the answers are quite different. The farther out you shoot, the bigger difference you will see between the two methods.

P.S. What is "Magilla?"

Truth and Objectivity are Mutually Inclusive

S1, Thanks for welcoming me in. Well, a "Magilla" (Ma Gilla; W’s Dictionary) is somewhat short for allot of stuff going on. I hear what you are saying, (Loudly) and appreciate your methodology. I also appreciate Sierra’s Infinity software, Mr. McDonald's article as well as using Exbal on the Palm, and have never had a problems hitting at distances beyond 1000 yards and on an angle.

But, as far as locating the Cosine or Angle of Inclination, I prefer utilizing the ACI instead of a Theodolite or protractor.

http://www.snipertools.com

W......What solution does your method give you for an 800 yard shot with a 175 going 2660 with a scope to borline diff. of 2.6" shot at 6,000 ft. ASL, 40 degree incline?

How does your device calibrate for a slow shooting bullet with a real high B.C. verses a low B.C. bullet with a super high muzzle velocity?



Truth and Objectivity are Mutually Inclusive

[This message was edited by S1 on Thu February 19 2004 at 9:21.]

S1: I just took a break from packing as I leave for Shot tomorrow. Let see whats' up here... O.K.

"W......What solution does your method give you for an 800 yard shot with a 175 going 2660 with a scope to borline diff. of 2.6" shot at 5,000 ft. ASL, 40 degree incline?"

"How does your device calibrate for a slow shooting bullet with a real high B.C. verses a low B.C. bullet with a super high muzzle velocity?"

First of all, what type of bullet? Secondly, 5,000 feeet above see level has a large barometric varience. I am perceiving this as a lesson to be learned? S1, as a shooter, long range shooter, are you familiar with how sunlight effects your bullets poi? How low will the poi be with the sun behind you? How far to the right will the poi be with the sun to the right of you? Altitude is altitude, and with all of the different corresponding BP changes comingled with Density altitude etc.. Your morning met data is going to be diferent then your afternoon met data. My log books go back to 1973 and every time a variable changed on a rifle, ie barrel, and even time of year, all of the data changed. I too prefer to elliminate as much geusswork as possible. Based upon my data / experiences comingled with software that utilizes up to date ballistic tables ie. (Exbal and Infinety). Thats what I would use to get you the data that you asked for. However, I could not get you a quantified answer because of the other variables and non constants involved. So, if you are looking for a finite answer, it is impossible to deliver.

The device is utilized as the preferred method taught in all military and federal presicion shooting schools. It is not an absolute but it does work well, very well. In fact there was a new world record made with it. (2,583 meters with a .338 Lapua). It happened on another continent this past year. I think that you would find it Very useful.


http://www.snipertools.com

[This message was edited by W on Tue February 10 2004 at 18:31.]

[This message was edited by W on Tue February 10 2004 at 18:32.]

[This message was edited by W on Tue February 10 2004 at 18:41.]

W....We use things like Vector IV's and even more compact units to determine range and inclination. I like your device and hope you sell a bunch at the shot show. Make no mistake, this is not my first Rodeo, I have done my homework on inclination angle shots, and made kills past 2600 yards on inclines. Multiplying range by cosine does not give a workable answer for long range precision shooting.

Truth and Objectivity are Mutually Inclusive

S1: Good morning. Well, I can tell by the way that you think that we would probably make good shooting buddies. You wouldn't by any chance be one of the long range guys from the Reno area? In any regards, my best to you and thanks for your good wishes.

http://www.snipertools.com

Nope, live and work in Missouri. Furthest west I get is Sandia Nat. Labs. Would like to talk with you about adding functionality to your device when you get back from the show, email me at s1@artactical.com when its over.

Truth and Objectivity are Mutually Inclusive