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Refining Ballistic Reticles??

 
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Old 02-06-2005, 08:07 PM
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Refining Ballistic Reticles??

I posted this awhile back on the specialty pistols forum, and thought it might be of interest to some here:

Sometimes the trajectory curve doesn't follow ballistic reticles well enuf to give even 50 or 100 yd. zero's, but it's actually fairly easy to apply a little trick that the mil-dot guys use to reference their zeros in 25, or 50 yd. increments.
OK, suppose we have a 2-7X Burris that has the following stadia subtensions in MOA:
2.1
6.2
10.4
15.5

Now, run a ballistics program for your rig-- say a hypothetical 308 Encore-- 150 Ball. Tip @ 2600 fps @ 4600 elevation- 50 degrees, 200 yd sight-in distance. The stadia zeros will be:

272 yds.
393
502
618

Now most guys will just leave it at that, or maybe tweak it a little to try and get a better trajectory-stadia fit to get more even (50 or 100 yd.) zero #'s, and then simply approximate their holdovers for in between ranges, but suppose u want to leave it right where it's at. It's not a bad "fit", but may not be the easiest to try and shoot a coyote @ 550, or 459, or whatever range. But if there was a way to reference a little more accurately for interpolating between stadia, then it would provide a little edge, so to speak, for better long-range hits. As mentioned before, some of the mil-dot users apply a system that helps for interpolating by dividing the stadia (mil-dot) gaps into tenths of a unit, thereby making interpolation easier instead of just guessing. Turns out that system can be easily applied for our ballistic reticles also like this:

1) Subtract each stadia MOA measurement from the next larger stadia to calculate total MOA gap between each stadia:

6.2 - 2.1= 4.1 MOA
10.4 - 6.2= 4.2
15.5 - 10.4= 5.1

Now we already know that 2.1 MOA (our 1st stadia)= 272 yds., and the 2nd stadia= 393 yds., but we want to know where 300, 325, 350, and 375 lie between those 2 stadia, so, if we refer to the trajectory printout, we see that the following drop in MOA for each range is:

300= 2.75
325= 3.75
350= 4.5
375= 5.25

Now subtract the closer STADIA MOA zero from each 25 yd. MOA calc, as follows:

300= 2.75- 2.1= .65
325= 3.75- 2.1= 1.65
350= 4.5-2.1= 2.4
375= 5.25- 2.1= 3.15

Now simply divide each remainder by the total gap MOA calc. (4.1), to get the amount of interpolation for each range between the 2 stadia (what we're doing is making imaginary stadia that's easier to reference besides just guessing for each 25 yd. range increment), as follows:

300= .65/4.1= .2
325= 1.65/4.1= .4
350= 2.4/4.1= .6
375= 3.15/4.1= .75

Now calculate the rest of the 25 (or 50-- whatever u choose) yd. zeros for the other stadia "gaps", and make a "better" range sticker for each zero. The part we just calculated would look like this on the range sticker:

272= 1 SU (stadia unit)
300= 1.2
325= 1.4
350= 1.6
375= 1.75
393= 2

I would venture to say that a guy using this system could become very proficient at placing the bullets right where they need to go, with a little prtactice-- even at long-range.
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