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Help with a scope? I think it is really a MOA question

 
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  #1  
Old 11-08-2006, 01:39 PM
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Help with a scope? I think it is really a MOA question

I am trying to figure out the range marking on a Burris Ballistic Plex scope. The website says that the first mark is 1.5" at 100 yards. The others are 4.5, 7.5, and 11 inches at 100 yards. Would that mean 200 yards that would compensate for a 3" drop from the 100 yard zero. (1.5" x 2= 3"). If so that would mean 4.5x 3= 13.5" at 300 and 7.5 x 4= 30 at 400 and so on right? I admitt I barely understand the MOA system. Help would be greatly appreciated. And if I am totally wrong please excuse my ignorance, I just know enough about long range shooting to know I dont know sh*t
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  #2  
Old 11-08-2006, 03:27 PM
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Re: Help with a scope? I think it is really a MOA question

Cody Perkins,
<font color="blue">Let me try to help. The following photo I got it from Burris' Website. </font>

[ QUOTE ]
Would that mean 200 yards that would compensate for a 3" drop from the 100 yard zero.

[/ QUOTE ]
<font color="blue">This first part is right! When using the 200 yard mark it will compensate for the 3 inches drop from a 100 yards zero. </font>
[ QUOTE ]
(1.5" x 2= 3"). If so that would mean 4.5x 3= 13.5" at 300 and 7.5 x 4= 30 at 400 and so on right?

[/ QUOTE ]
<font color="blue">This is also correct mathematically speaking, but a more conventional approach would be (3/2=1.5") which is telling you that if zeroed at 100 yards and it drops 3" at 200 yards then you need to correct for 1.5" at 100 yards which would be 6 clicks when using a 4 clicks to an inch turret. I know you would not be dealing with turrets but at least you would be following the more usable conventional approach.
Let me give your an Example: Let's say you zero your rifle at 100 yards and it's dead on. Now you place a target at 500 yards and carefully you shoot many three shots groups and when averaging them you come to the conclusion that the rifle using the 500 yards mark is shooting 4" low at that distance. Now let's say you're more interested in being dead on at 500 yards using the 500 yards mark on the scope than being dead on at 100 yards using the center of the scope. So our next thing to know would be how many clicks do I move the scope up to be dead on at 500 yards? The answer using the conventional approach would be: (4 / 5 = 0.8") meaning that a drop of 4" at 500 yards needs a correction of 0.8" at 100 yards. Therefore, if you have 4 clicks to one inch, (0.8 * 4 = 3.2 clicks), now move 3 clicks up and you'll be dead on at 500 yards and only 3/4" high at 100. That'd work for me! [img]/ubbthreads/images/graemlins/smile.gif[/img] </font>
[ QUOTE ]
I admitt I barely understand the MOA system.

[/ QUOTE ]
<font color="blue">To the best of my understanding, none of it is dealing with the MOA system, Burris is just interested in how many inches it drops at 200, 300, 400 and 500 yards and their respective correction at 100 yards. </font> <font color="blue"> </font>
[ QUOTE ]
And if I am totally wrong please excuse my ignorance,

[/ QUOTE ]
<font color="blue">You're not wrong, I'm just trying to re-direct our thinking and why. If I'm wrong I know someone will correct me but for now all you need is to zero at 100 and using the other marks shoot at the other distances. If you're close enough for big game hunting then be happy! [img]/ubbthreads/images/graemlins/smile.gif[/img] Good Luck to you!!! </font>
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  #3  
Old 11-08-2006, 03:30 PM
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Re: Help with a scope? I think it is really a MOA question

Cody, you've got it, won't be long and it will be second nature.

I've been looking at a Fulfield with the balistic plex for a shorter range (400 yds) gun that I can paint camo. Just can't bring myself to paint a Nikon tactical. I'm just not sure I'll find the glass acceptable.

Chris
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  #4  
Old 11-13-2006, 02:35 AM
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Re: Help with a scope? I think it is really a MOA question

I contacted Burris and asked what MOA the extra crossmarks are set at. They acted like I was from Mars. So, I did a little research and found that 1 MOA= 1.047". I divided the inch correction at 100 yds by 1.047 and came up with the moa for ea hashmark. They are: 1.5, 4.29, 7.2 and 10.5. Any trajectory chart that prints out a MOA reading can now be used to calibrate the scope to the gun. I'll try and see if quick target posts here......
Tabular trajectory data at Std.ICAO Atmosphere
-------------------------------------------------------------------------------------
Gun / Ammunition : 7mm
Bullet : .284, 154, Hornady SP 2830
Bullet weight : 154 grains or 9.98 Grams
Muzzle velocity : 3100 fps
Crosswind speed : 10 Mph
Ballistic Coefficient(s) (G1):
C1=0.433@V&gt;0 fps;


Optimum trajectory information :
Optimum sight-in range (X) = 205 Yds.
with max. ordinate above LOS at range (M)= 122 Yds.
and max. point blank range (P)= 238 Yds.
-------------------------------------------------------------------------------------
Sight-in clicks, 1 click = 0.992 cm/100 m or 0.391 in/100 m
Height of sight above bore axis = 3.81 cm or 1.50 inch
Gun is zeroed-in at 250 yds,
by sighting-in at level firing
-------------------------------------------------------------------------------------
Range Velo Time of Energy Path Deflection Total Sight correction Target
city flight to at crosswind drop for setting new lead
LOS of 10.0 Mph zero range 33 fps
-------------------------------------------------------------------------------------
ˇYards fps s ft.lbs. in. in. MOA in. Clicks MOA yds ˇ
-------------------------------------------------------------------------------------
| 0 3100 0.0000 3286 -1.5 0.0 ----- 0.0 ------ ----- 0.00
| 20 3054 0.0195 3190 -0.4 0.0 0.09 0.1 +5.9 +2.01 0.21
| 40 3009 0.0392 3096 +0.5 0.1 0.19 0.3 -3.6 -1.22 0.43
| 60 2964 0.0592 3005 +1.3 0.2 0.31 0.7 -6.0 -2.05 0.65
| 80 2920 0.0795 2916 +1.9 0.4 0.44 1.2 -6.7 -2.28 0.87
| 100 2876 0.1002 2829 +2.4 0.6 0.58 1.9 -6.6 -2.26 1.10
| 120 2833 0.1213 2744 +2.6 0.9 0.73 2.8 -6.2 -2.11 1.33
| 140 2790 0.1428 2661 +2.7 1.3 0.87 3.8 -5.5 -1.88 1.56
M 142 2785 0.1449 2653 +2.7 1.3 0.89 3.9 -5.4 -1.86 1.58
| 160 2747 0.1645 2581 +2.7 1.7 1.02 5.0 -4.7 -1.60 1.80
| 180 2705 0.1867 2502 +2.4 2.2 1.17 6.4 -3.8 -1.29 2.04
| 200 2663 0.2091 2425 +2.0 2.7 1.31 8.0 -2.8 -0.95 2.29
| 220 2622 0.2319 2351 +1.3 3.3 1.45 9.8 -1.7 -0.58 2.54
| 240 2581 0.2550 2278 +0.5 4.0 1.59 11.8 -0.6 -0.20 2.79
X 250 2560 0.2666 2242 0.0 4.3 1.66 12.9 0.0 0.00 2.92
| 260 2540 0.2784 2206 -0.6 4.7 1.73 14.0 +0.6 +0.20 3.04
P 276 2508 0.2973 2151 -1.6 5.3 1.84 15.9 +1.6 +0.53 3.25
| 280 2500 0.3020 2137 -1.8 5.5 1.86 16.4 +1.8 +0.61 3.30
| 300 2460 0.3259 2069 -3.3 6.3 1.99 19.0 +3.0 +1.03 3.56
| 320 2420 0.3503 2003 -5.0 7.2 2.14 21.8 +4.3 +1.47 3.83
| 340 2381 0.3753 1939 -6.9 8.1 2.29 24.9 +5.7 +1.93 4.10
| 360 2342 0.4007 1876 -9.1 9.2 2.44 28.3 +7.1 +2.41 4.38
| 380 2304 0.4266 1815 -11.6 10.4 2.60 31.9 +8.5 +2.90 4.67
| 400 2266 0.4530 1756 -14.3 11.6 2.77 35.8 +10.0 +3.40 4.95
| 420 2228 0.4798 1698 -17.3 12.9 2.94 39.9 +11.5 +3.92 5.25
| 440 2191 0.5071 1641 -20.6 14.3 3.11 44.4 +13.1 +4.46 5.55
| 460 2154 0.5348 1586 -24.1 15.8 3.28 49.1 +14.7 +5.00 5.85
| 480 2117 0.5630 1533 -28.0 17.3 3.45 54.1 +16.3 +5.56 6.16
| 500 2081 0.5915 1480 -32.2 18.9 3.62 59.4 +18.0 +6.13 6.47
| 520 2045 0.6205 1429 -36.6 20.6 3.79 65.0 +19.7 +6.72 6.79
| 540 2009 0.6498 1380 -41.4 22.4 3.96 70.9 +21.4 +7.31 7.11
| 560 1974 0.6793 1332 -46.4 24.2 4.12 77.1 +23.2 +7.90 7.43
| 580 1938 0.7095 1285 -51.8 26.1 4.30 83.6 +25.0 +8.52 7.76
| 600 1904 0.7406 1239 -57.6 28.1 4.48 90.6 +26.9 +9.16 8.10
| 620 1869 0.7724 1195 -63.9 30.3 4.67 98.0 +28.8 +9.83 8.45
| 640 1836 0.8050 1152 -70.6 32.7 4.88 105.9 +30.9 +10.53 8.80
| 660 1803 0.8383 1111 -77.8 35.1 5.08 114.2 +33.0 +11.25 9.17
| 680 1770 0.8723 1071 -85.5 37.7 5.30 123.0 +35.2 +11.99 9.54
| 700 1738 0.9070 1033 -93.6 40.4 5.51 132.3 +37.4 +12.76 9.92
| 720 1706 0.9423 995 -102.2 43.2 5.73 142.0 +39.7 +13.54 10.30
| 725 1698 0.9512 986 -104.4 43.9 5.79 144.5 +40.3 +13.74 10.40
-------------------------------------------------------------------------------------
M = Peak vs. L.O.S, X = Set Zero, P = Max. Point Blank Range
Elevation above Angle of Site (0.0 deg.) = 0.0916 deg.


What you do now is go to the 2nd column from the right and match the closest number to the moa mark on the scope. Then you look to the left and see what yardage that mark corresponds to. I wish I knew how to make the numbers in color so they come out more plain but......this would be the estimate. 1st notch....320yds......2nd 430 yds, 3rd 535yds, and top of post 640yds.
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  #5  
Old 11-14-2006, 07:54 AM
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Join Date: Dec 2005
Posts: 2,114
Re: Help with a scope? I think it is really a MOA question

For what it's worth, across several internally adjusted scopes of the same make and model, one can easily find a few percent differences in how far a given change on the adjustments move bullet impact. That's 'cause lenses used in optics have tolerances in their focal length. Two percent is common and a one percent tolerance is rare and costly. When one adds up all the tolerances, it's going to explain why the physical image size at the adjustment plane varies across several scopes. Which means a given adjustment change will result in a smaller bullet impact change for a larger image (from longer focal lengths) at the adjustment plane than a smaller image (from shorter focal lengths).

The mechanics across a given make and model have close to zero tolerance. A given thread count and length of the erector tube between pivot and adjustment point are fixed. So these parts will give the same amount of movement per click/mark; lost motion and slop excluded.

The only scopes that have exactly the same movement per click are the externally adjustable ones having mounts spaced the same distance apart. Lens focal length doesn't come into play with these as its simple mechanical movement and uniform threads per inch of the adjustments that make 'em this way.

The result of lens focal length and pivot arm length of the erector tube combined with exactly how far the mechanics move per click/mark determines the movement. It ain't always exactly 1- or 1.0472-inch (whatever you believe is correct) at 100 yards per MOA of adjustment change. Again, only the externally adjusted scopes moving exactly .002-inch on a 7.2-inch mount spacing that moved impact exactly 1 inch at 100 yards per MOA of change. Makers of internally adjusted scopes may quote some movement per adjustment, but manufacturing tolerances rarely make it so.

It pays to determine your scope's impact change per click/mark of adjustment. 'Tain't easy, but can be done and will resolve a lot of problems when "exact" stuff is needed.
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