Optical Center, canted bases, mounting questions....

Discussion in 'Long Range Scopes and Other Optics' started by cdherman, Aug 29, 2008.

  1. cdherman

    cdherman Well-Known Member

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    I have a Zeiss 4.5x14 Has 64 MOA updown. Going on top of 270 WSM w/26" bbl. I think I can get a 130 grain bullet to 3300 f/s, maybe 3400 with the long bbl.

    I have not yet graduated to true, long range shooting, but decided to set things up so that I could at least shoot a little longer range if desired.

    I have purchased Burris signature bases with a full set of inserts so I can adjust the cant of the bases. Using a combination of inserts, the Burris setup can go from 5 moa to 30 moa cant.

    The question is how much. I read a lot about the importance of optical center. So that argues for no more cant than is absolutely necessary.

    I looked at the balistic tables for my load. Wow, beyond about 500 yards it gets pretty nasty. No wonder you fellas trend toward cannons...

    My drop at 500 yards is about 34 inches. At 600 it 55. Seems like to do me any good at all I'd need to put a full 20 or even 25 MOA cant in the bases.

    How much is that going to effect my optical performance when I shoot at close range though? Presumeing I keep a 100 yard zero and shoot at a deer at 100 yards, the scope would be at the very bottom of its adjustments. Totally outside of its optical center.

    Appreciate your thoughts -- I intend to sight it this weekend and hope to cinch down the mounts and forget about them.
     
  2. Charles A

    Charles A Well-Known Member

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    Zero your rifle with 0 degrees on the rings. Find out how many MOA you lost for that 100 yard zero, then use the ring inserts to gain that back leaving 5 MOA or so. You want "optical center" to be at the farther ranges, not close ones.....
     

  3. AJ Peacock

    AJ Peacock Well-Known Member

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    +1 Charles A


    AJ
     
  4. Charles A

    Charles A Well-Known Member

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    Also, stop thinking in inches. Start thinking in MOA.
     
  5. Ridge Runner

    Ridge Runner Well-Known Member

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    55" is 8.75 MOA at 600 yards, you have 32 MOA up from center, put 20 MOA worth of inserts in, your still 12 MOA from the down stop. and therefore have 52 MOA of up left, a 150 gr .277 ballistic tip at 3200 fps, sighted in at 300 yards only needs 21.5 MOA to make 1K and is just under 3" high at 100.
    RR
     
  6. cdherman

    cdherman Well-Known Member

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    As I read these replies I of course had an "ah-ha" moment. I am not un-educated nor usually considered dumb, but I had completely failed to recognise what I actually already knew. To correct for the 55 INCH drop at 600 yards far fewer clicks are needed that at 100. I knew that and I now KNOW that. Sometimes 2+2 only equals four after someone reminds you.........

    I am knew, please forgive.

    I suppose I should go search, but perchance -- anyone have a nice link to a balistic calculator that can give me the number in MOA so I can start thinking in terms of what I can (or cannot) dial-in?

    Thanks for the advice so far. Appreciate it....
     
  7. AJ Peacock

    AJ Peacock Well-Known Member

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    Do a google for "jbm ballistic calculator" and that should get you started.


    AJ
     
  8. ss7mm

    ss7mm Writers Guild

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  9. davewilson

    davewilson Well-Known Member

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    i know you want to tighten those rings and forget them but you also have to test the scope to see if it's tracking verticle or not. put a target up at 100, make sure you have at least 30-36" of usable material above it. shoot 2-3 shots into target, hopefully you'll have a nice small group in the center left and right. take a level and mark a perfect verticle line from the center of your group up at least 30". now go back and crank your scope up 20 MOA or so and shoot another group. this group should print 20 or so inches above the first and on the verticle line. if it doesn't, you have to rotate your scope till it does. now you can tighten those rings down and practice, practice, practice!