I printed your post out for study. You lost me. [img]/ubbthreads/images/graemlins/wink.gif[/img]
Rifleman's Rule" does work (as you mentioned)
It's the "Rifleman's Rule" that is put forward on the ACI site that is sorely inaccurate.
55 grain Sierra BlitzKing (.224")
1.8" scope height
0 mph wind
30 degree angle
250 yd Zero
65 degrees F
966' above sea level
Here's what I get using JBM Trajectory Calc:
400 yards path -12.3" 2.9 MOA (1/4 minute)
400 yards path w/30 degree angle -10.6" 2.5 MOA
ACI/using RR (cosine .87 X 400 = 348 yards) -5.6" 1.5 MOA
ACI/using IRR - .87 X 2.9 = 2.5 MOA
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<font color="red"> Now, this is an easy adjustment; a simple equation that will put you very close to right on target, if not dead on. However there are two ways to obtain this and one is a little more accurate than the other.
I think what Ward is refering to here as being more accurate is the method that is more comonly used, by Sierra and most every ballistic program I've run accross... including Exbal.
1.0 minus the Cosine of the incline/decline angle multiplied by the actual drop from your boreline (not to be confused with bullet path). Subtract this amount from your level fire come-up.
First - It's easiest to do this math in MOA verses inches then converting to MOA.
770 yard shot at a 35 degree angle.
LOS to target is 770 yards, and corrected horizontal range is meaningless here.
35 deg = .819 Cosine
1 - .819 = .181
"Vertical" drop from boreline = 16.32 MOA at 770 yards - (this remains constant, level fire or incline fire)
"Level fire" bullet Path = 10.63 MOA at 770 yards.
16.32 MOA * .181 = 2.954 MOA
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.
The easiest thing that I have done to help simplify these calculations in the field was to ask Jim Ristow at RSI ( www.shootingsoftware.com
) to modify the drop from boreline results in his program to read in MOA, not just inches etc... thanks Jim!.
What you can now see by having the DROP and PATH column results "both" in MOA is the exact MOA "difference" between the two.
The reason this becomes important is that in the field we use our bullet path - not the drop from our bore line, which is what we now find is needed for these calculations to be ACCURATE.
If you compare bore line DROP and bullet PATH MOA, you'll notice that there might be an average of 4.0 MOA difference between the two from 300 yards to 1200 yards, or something like that... maybe it's 3.75, 5.5 MOA or some other number. The important thing is to know "what number" it IS to add to the bullet PATH numbers you have commited to memory already so you know the bore line DROP in an instant also.
This eliminates the need to carry or memorize a bore line DROP chart also... the point of this whole thing, it eliminates a step in the math involved with virtually no loss of accuracy of the firing solution.
If my PATH requires 10 MOA correction at 750 yards, and a 35 degree incline is encountered, I add my "set" 4 MOA to the 10 MOA for 14 MOA, then multiply by .181 for 2.5 MOA to subtract from the original 10 MOA.... so I dial 7.5 MOA.
Pretty simple really, but very ACCURATE.
Another method, much more accurate than the "Slant Range converted to Horizontal Range" method (which our military has embraced), but still lacking for accuracy is where you multiply the bullet PATH MOA correction by the cosine of the incline angle and simply dial this new MOA solution.
Sierra explains this all much better than I can on their site here. http://www.exteriorballistics.com/eb.../article1.html
They also explain in detail WHY, and WHAT is going on that effects incline fire.
Hope that helps and I didn't confuse.
Here's another discussion from last year too.
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