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Ballistic mil dot question

 
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  #1  
Old 05-08-2005, 06:00 PM
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Posts: 50
Ballistic mil dot question

I have an 8x32 burris black diamond with the ballistic mil dot. I am curious if there is any information that would tell me what the marks would reprsent for drop at any desired magnification, instead of just the single factory calibration mark. Thanks in advance, cdog.
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  #2  
Old 05-08-2005, 08:33 PM
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Re: Ballistic mil dot question

As I understand their system all you need to do is to apply a ratio multiplier. which works on the following principle:
The scope is set up at 14 power. The marks have half as much drop at 28 Power (14 divided by 28 = 0.5). The marks have twice the amount of drop at 7 power (14 divided by 7 = 2)

http://www.burrisoptics.com/reticles.html

At 14 X adjustment for drop in inches is
0 =0
1=0.8
2=2.4
3=4.6
4=7.2
5=10.4
6=14.5
This is adjustment not a drop

At 20 X simply multiply by 14/20 to each of the numbers above to get what the adjustment would be at that magnification. To get the drops just multiply by adjustment by range factor i.e. MOA calculation.

Let us say you are shooting a different bullet or different caliber than Burris specifies then you would get the drops for your bullet and work backwards to get the MOA adjustment and then ratio it with the 14X MOA to get the correct magnification setting.

Now then all of the above was just theory. In reality you will not be able to get a good match to a different bullet becuase the hash marks represent a particular "shape" of curve and changing magnification does not change the "shape" of the curve. This is not exactly right, but as long as "you, the shooter" cannot move the hash marks relative to each other then options are limited.

I doubt that I have done a very good job of expalining what I am trying to say and it is getting late. I may have made a mistake somewhere in the above because I did not do very many calculations to check my assumptions. I will think about this some tomorrow.

If you want some clarification or if I am not answering what you asked just say so. There is about 20 minutes worth of math in my head that I can add tommorrow to show you what I mean.
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  #3  
Old 05-09-2005, 07:32 PM
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Re: Ballistic mil dot question

Cont'd

Let us think about how the hashmarks work at different magnification. we will use the 300 yard mark as an example.

For 1000 yards

Frist two hash marks

At 14 power

2.4 inches @ 100yds X 1000yds/100yds = 24 inches @ 1000yds

At 28 X

14/28 = 0.5

2.4 inches X 0.5 X 1000/100 = 12 inches

At 32 Power

14/32 = 0.4375

2.4 inches X 0.4375 X 1000yds/100yds = 10.5 inches @ 1000 yds


A 8 X

14/8 = 1.75

2.4 inches X 1.75 X 1000yds/100yds = 42 inches


What we see here is that at the higher magnification that we would like too make the longer shots the hash marks are not of great use. At lower power they are more use but we will have a harder time with our aiming point because of the low power.


Let us see if we can adapt the hash marks to another bullet


Being as the hash marks are “calibrated at 14” for the 300 Win mag w 150 grain bullet @ 3350fps let us see what happens is we shoot the 220 Sierra Mk at 2700 fps

The reticules is calibrated for the following drops from 100 yd zero : 200 = 1.6, 300= 7.3, 400= 18.3, 500= 36.0, 600 = 62.6 for the 150 gr

According to Sierra manual the 220SMK for a 100 yds Zero drop at range will be 200 = 3.79, 300= 13.48, 400= 29.77, 500 = 53.45, 600 or = 85.44
In other words at 600 yds if we use the hash mark at 14 power we will undershoot the target by 85.44 - 62.6 = 22.8 inches. We will miss an elk, we will miss a moose we will miss a buffalo and we might hit an elephant but only wound it and it will stomp on us.

Let convert the 220 SMK trajectory to inches adjustment

200 = 1.9
300= 4.49
400= 7.44
500= 10.7
600= 14.26

What we want to do is see if there is a power setting that will allow us to use the hash marks with the 220SMK, so let us calculate a ratio of 600 yd adjustments

14.26 divided by 10.4 = 1.37 is the ratio

14power divided by 1.37 = 10.2 power will be the calibration point .

OK then let us compute a table to double check our work


0 =0
200yd=0.8 X 1.37 = 1.096
300yd=2.4 X 1.37 =3.28
400yd=4.6 X 1.37 = 6.3
500yd=7.2 X 1.37 =9.86
600yd=10.4 X 1.37 =14.25

What we see is that by using the ratio at 600 yds we get a good match at 600 yds but at 200 yds we will shoot 2 inches too low and at three hundred yards we will shoot 3 inches too low and at 400 yards we will shoot 4.4 inches low and at 500 yds we will be 4 inches low. Now then if we choose to run the ratio at 500 yards instead of 600 yds the following will happen

10.7 divided by 7.2 = 1.49

14 power divided by 1.49 = 9.4 power calibration point

200yd=0.8 X 1.49 = 1.19
300yd=2.4 X 1.49 = 3.58
400yd=4.6 X 1.49 = 6.85
500yd=7.2 X 1.49 =10.7
600yd=10.4 X 1.49 =15.5

At 200 yds we will be 1.4 inches low, at 300 we will be 2.2 inches low, at 400 we will be 2.4 inches low, at 500 we will be dead on and at 600 yds we will be 7.2 inches high. This is not good to be 7 inches high so we can see it is better to run ratios at 600 yards than 50 yards.

Now then as we all understand scope manufacturers round off their numbers and when they say the calibration point is about 14 power we do not know what that means. It could be 14.3 which would start to change the calculations.

Now then I hope you are not too confused with all of thess calculations and if you had a different question in mind than just ask it again differently.
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  #4  
Old 05-09-2005, 08:18 PM
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Join Date: Dec 2004
Posts: 50
Re: Ballistic mil dot question

Thank you for your explanation. I beleive i understand what you are saying, in that i should learn to use my clicks or purchase this reticle in a first focal plane scope, if such a toy can be had, i wonder why it isnt? This is my first "real" scope ever and i wanted something easy and hoped that i could match the drop of this reticle by rechambering my 223 to a 22br. Thanks again,cdog.
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  #5  
Old 05-10-2005, 08:05 AM
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Location: Potomac River
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Re: Ballistic mil dot question

The built in drop compensators have been around since the 1970's. Bushnell had a "BDC" bullet drop compensator. In those days I was shooting a Ruger #1 25-06 with a Redfield accurange/ accutrac and this was acceptable accuracy for deer sized animals to about 500yds. When I started going beyond 500 yds I left the BDC's behind and began using target turrets and spinning the dials. I have a Burris ballistic plex on a inline muzzle loader because this is not an accurate gun and I am not trying to hit a small target and for this type of greater than MOA gun and deer at 300 yds the level of accuracy is acceptable.

If you like "stuff" in your reticle and want to play around with a mil dot system here is a little game.

http://www.shooterready.com/lrsdemohi.html

A word of advice. Use the calculator built into the game other wise you will shoot hundreds of rounds and not score well. If you need the equations for using the calculator come back here and I will post it for you. It is similar to what I did above.
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  #6  
Old 05-14-2005, 10:56 PM
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Join Date: Dec 2001
Location: Pennsylvania
Posts: 230
Re: Ballistic mil dot question

I had the same scope. I taped a ruler at 100 yards and read the values in inches of what the hash marks referenced at 100 yards. For that scope, when set to 32 power. The first hash was .25", the second 1.25, 3rd 2.375, 4th 3.625, 5th 5.375, and the post point, 7.5. You could do the same at any power but I went to the common point of "max".

With the known value at 100 yards you can then apply data from any number of ballistic programs to determine which line to use. Using the points that I measured above the second hash is 1.25 inches. Plug a 1.25 inch high at 100 reading into RCBS external ballistics calculator or equivalent and when the bullet path crosses line of sight, write that down. This is your yardage for that point.

With that scope for example with a 22-250 3700 fps, 55gr NBtip the first hash was 147yd, 2nd 227, 3rd 293, 4th 355, 5th 428, post 505.

Good Luck

Edited Note: I misread. This was a Burris Signature Series 8-32, and not the Black Diamond so I am not sure if the Bal-Mil Dot is the same as the above figures.
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  #7  
Old 05-20-2005, 11:25 PM
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Location: Pueblo, CO
Posts: 1,175
Re: Ballistic mil dot question

Pretty good BuffaloBob-- the system is based on a ratio and proportion calculation, since as magnification changes so does reticle subtension-- BUT it's an inversely proportional formula since as magnification INCREASES, reticle subtension DECREASES-- and it's all right here--

www.ottllc.com/specialtypistols/sp20.pdf

under 4A-- subtension changes with magnification changes.

We actually used this system in the field once to shoot pr. dogs @ 250-300 yds. with a kids 22 LR-- and it was just an approximation on his plex post tip reticle subtension (which turned out to be right on luckily), at 6 MOA @ 9X, and 18MOA @ 3X on a 3-9X Simmons.
Hope it helps some
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