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Posted: 4/19/2017 10:54:40 PM EDT
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I know the formula is out there and I cant work my way back (I know i should have paid better attention in algebra)
I have a 30-30/duplex scope with a fixed 2.5x magnification I understand the standard is a 30" spread from apex to apex on 4x power. Ive worked it out to get the estimated yardages where the spread is at 15", 30", 45", and 60" What im having trouble doing is getting nice round yardage numbers at 25, 50, 75, 100 and figuring out what the spread is at those distances Here is what I have incase ive botched that math as well: 15" spread @ 31.25 yd 30" spread @ 62.50 yd 45" spread @ 112.5 yd 60" spread @ 125.0 yd FWIW im setting this up on my ranch truck AR for coyote/hog/wild dog. The reticle also has a ring that splits each axis into 1/4's for a bit more accurate reading on this smaller game |
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Math won't work for several reasons. Too many variables exist even if you got the math right.
I zero every .223 high power rifle I own at 200 yards with my standard load of 24.5 grains of Varget with 69 grain Sierra Match Kings seated at 2.250". Depending on barrel length I am 1.5" (20" barrel) to 3" high (11.5" barrel) at 100 and 7.5" (20" barrel) to 14.5" (11.5" barrel) low at 300. Each rifle is different based on muzzle velocity. Everything changes if I switch bullet brands, powder charges or bullet weight. I find it easier and more accurate to put click adjustments on the scope than to use Kentucky windage or elevation. I buy scopes that allow me to do this easily. 200 yard zeroes allow me to point and shoot inside 230 yards without compensating for elevation. |
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You should be able to use your duplex scope for range finding.
It is much easier with a fixed power scope. I'd set up a target with a 1" grid on it at 50 yds and see what your spread is between the duplex points. It's linear from there. If your reticle is reading 12" at 50 yds, then it is 24" at 100 etc. Then you just have to know the approximate size of the target to estimate range. If you have an 8" tall gopher standing out there filling 1/2 the duplex he is 67 yds out. As far as sighting in though with a scope without turrets I think you have to decide on your point-blank range. What I do with my big game rifle: I have it set to maximize what I consider to be point blank - i.e. for elk I want to hold center and have no more than +/- 3" deviation in the bullet trajectory. That gives me about 300 yds with my rifle I don't have to do anything but hold dead center and I'm good. If you have time you can english it a bit, like I know I'm under 150 yds so I can hold a couple inches low. More difficult with varmints where your kill zone may only be a couple inches, especially with something like a .30-30 that isn't particularly flat shooting. Your point blank range for hold center and hit within +/- 1 inch is not very far. I also have a trajectory table I made up for ranges assuming i'm holding on top of the lower post of the duplex - basically holding 6" high at 100 with the scope on 9x. |
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In the 8" gopher example - he's filling up 1/2 the duplex at 8" - so the entire duplex is covering 16". In my example I made up the fact that your duplex is covering 12" at 50 yds.
it's 'similar triangles' at this point. take the ratio of the heights * known distance = estimated distance. (16/12)*50 = 67. It's accurate - if you have a pretty good grasp of the size of the thing you're ranging. On my little cheat sheet I have written down the average depth of an elks chest, a mule deers chest, an antelope chest, etc. It's not perfect, but I'm horrible at guessing distance - especially when we head out to the flatlands and try to shoot antelope. Damn things look like they are a mile away, I constantly overestimate the yardage if I'm just guessing at it. If I were you, I'd take my scope to the range and find a yardage that fits your duplex well and that you can remember. Make yourself a cheat sheet for critters you're likely to encounter. By the time you've built a cheat sheet you'll know your scope pretty well. |
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