Range Report Angle Compensation Calculations

[SLO]

I was reading The Ultimate Sniper by Maj. Plaster and he gave a calculation for Angle Compensation that yields significantly different results than using the angle cosine to multiply by line of site to get corrected distance. His formula is to multiply the standard drop (in inches) at the line of site distance by a factor that is based on angle (i.e. 25 degrees, the factor is .094). Here is an example of each:

<span style="font-weight: bold">Calculation using Cosine:</span>

Line of site distance: 550 yards
Angle: 25 degrees
Cosine: .91

550 x .91 = 500 yards corrected range

The Ballistics Charts show 83.4" of drop for FGMM 175gr at 500 yards

<span style="font-weight: bold">Calculation Plaster's Formula:</span>

Standard drop for FGMM at 550 yards: ~ 109"

109" x .094 = 10.246

So, this tells us that at 550 yards and 25 degrees, you would aim 10.246" lower than 550 yards at no angle.

So, if you take 109" and subtract 10.246", you get a total drop 98.754" at 550 yards at 25 degrees.

The second formula is obviously much less drop than using the cosine calculation. Does anyone know which is more accurate or have used the formula from The Ultimate Sniper? For what it's worth, it sites the Sierra Reloading Manual as the source of his formula.

 

[RobertB]

Re: Angle Compensation Calculations

I know some of his wind charts he gave on his website were way off. In a 10 mph wind, 900 yds had less drift than 800 yds. I still havent figured that out.

 

[Lowlight]

Re: Angle Compensation Calculations

Plaster is nothing more than a compiler of information, usually old and poorly vetted which is why his example is in Inches ... he gets a lot of stuff wrong in the Ultimate Sniper. It's really nothing more than a historical reference.

First off you have to make sure you are starting with the same drop from the same gun using the same bullet.

Then don't mix the Rifleman's Rule with the Improved Rifleman's Rule.

Most use the improved Rifleman's Rule which says, you take the cosine of the actual dope for "your rifle". So convert it to MOA or at least make sure you are using the same numbers in inches.

Taking the cosine of the yardage is the original Riflemans' Rule so you are mixing data points... yards vs drop.

With most targets it was mean to get a hit on Minute of Man or Minute of Mountain Goat, usually inside that 600 yards so you have a lot of fudge room. To get more precise, they added the Improved Rule, as the original was meant as a down and dirty quick fix. Sort of like the idea that when in doubt aim for the balls... or stomach, either way it will work on a full size target. But since those rules people realize there is a bit more involved if you want to get a precise hit on a sub moa target.

 

[RobertB]

Re: Angle Compensation Calculations

Im a flatlander so my angle shooting is not near what my hill brethren is. Is your angle correction different with flatter shooting rounds vs slower? Or is it always the "gravity"distance? Never really shot over a 10 or 15° i would quess. And that was when i really first started really learning this blessed passion of mine.

 

[Graham]

Re: Angle Compensation Calculations

Forget calculating inches of drop, it's not practical. Go to the source. Use the Improved Rifleman's Rule.
http://www.exteriorballistics.com/ebexplained/article1.html

 

[SLO]

Re: Angle Compensation Calculations

Thanks guys. In reading the Exterior Ballistics link that Graham posted, the Sierra method that is cited in The Ultimate Sniper seems to be more accurate than the Improved Rifleman's Rule

 

[SLO]

Re: Angle Compensation Calculations

So, I did a test with two different Ballistics apps i use for the iPhone and sure enough, the results were quite different when angles were applied. At 0 deg, both apps yield very similar results. When you get out to 600 yards and beyond and introduce angles more than 10 deg or so, the difference is significant.

Mildot Ballistics - with my FGMM 175gr profile at 800 yards and 35 deg, the drop calculates at -169.8" (-5.89 mil)
Ballistic FTE - with the same FGMM profile at 800 yards and 35 deg, the drop calculates -178.87" (-6.21 mil).

9" seems like a significant difference.

Now i need to determine which formulas each of them use.

 

[Lowlight]

Re: Angle Compensation Calculations

Sierra and Plaster are citing the "improved" rule, the old rule was to use the range and adjust the distance from 550 to 500... the new Improve rule adjust the dope for 550 as you still use the entire range for things like wind.

 

[Lowlight]

Re: Angle Compensation Calculations

Also it is probably something wrong in Mil Dot Ballistics, as Ballistics, BulletFlight and Shooter are all within .1 mils of the same answer...

Check your data make sure the environmental conditions are the same, the MV is the same and all the other data points.

Running it here, I am within a 1/10th of each other at 800 yards using 175gr going 2650fps.

 

[SLO]

Re: Angle Compensation Calculations

Thanks Lowlight! I double checked the conditions and zero was the same but current was different. So, that helped a little but now i have 5.89 vs 6.07. Anyway, sounds like MD Ballistics is a little off.

 

[Lowlight]

Re: Angle Compensation Calculations

5.89 can easily be 5.9 and 6.07 is 6 mils so there you are back to .1 to .2 mils which is no more than 3/4 MOA off of each other.

 

[SLO]

Re: Angle Compensation Calculations

I'm with you and agree. And, after running more comparisons, other angles are even closer in values. In fact, 30 and 25 deg yields the largest gap (which we have established isn't that much) but 0 - 20 deg and 35 - 50 deg are within .001 mil.

 

[00bullitt]

Re: Angle Compensation Calculations

What is the method that Gerald Perry uses that requires a bias number to formulate the solution?