wierd 3.5 MOA shift in zero

Discussion in 'Optics Talk' started by Jayne, Oct 20, 2018.

  1. Jayne

    Jayne Just here for the memes Charter Member Supporting Member

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    Last time I ran my AR was in a class in July, since then it's just been used for some dryfire drills but mostly sits in the safe. Today I got it back out to the range to test some loads, and found that the optic was wildly off. left/right was fine, but elevation was off by 5-6" at 100 yards. Once dialed in we played with various magnifications and didn't see a huge POI shift at 1x, 4x or 8x other than my eyeballs not seeing the same spots on the 50 yard test target.

    Vortex Strike Eagle 1-8x in the vortex mount. Hasn't shown any issues up to this point, about 3000 rounds through the rifle since mounting. Capped turrets, so nothing would have gotten bumped. Everything has lock tight on it and I verified nothing was in the slight bit loose when I checked the torque at home.

    Tried a 5 different loads (4 factory, 1 handload that works very well, btw), and they all shot about the same with respect to elevation being off.

    So, optic failing? I got it too hot at the class and warped something (420 rounds over the day with lots of pistol transitions and shoot house stuff so not too punishing on the rifle)? I shouldn't have cleaned it? It did have new gas rings in before the class, and I didn't check the zero before the class... so ? Can't have changed the velocity that much!

    Hand guard is free float and the screws are still torqued (and lock tighted).

    Ideas? Things to check?

    IMG_3340.jpg
     
  2. Zbizzle911

    Zbizzle911 Charter Freedom Lover Charter Member Supporting Member

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    Did you shoot the ammo you sighted it in with?

    I wouldn’t think 420 rounds would be enough to transfer the heat needed to mess the scope up.

    Remember banging it on anything during the class? Or how it was shooting in the class?

    What zero did you use? 50/200? Or something different?




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  3. Jayne

    Jayne Just here for the memes Charter Member Supporting Member

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    yes.

    I don't remember hitting it on anything, but we did do some work with the rifles sideways on top of things (barrels, barricades). Most of it was short range stuff and transitions so even if it was low I doubt I would have noticed.

    100 yards. I know it's not tacticool but it's what I did before the first class and I just left it so it's consistent with my other (non-AR) rifles.
     
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  4. Zbizzle911

    Zbizzle911 Charter Freedom Lover Charter Member Supporting Member

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    Hmmm that’s odd for sure. I guess just keep a check on it. I’m not judging your zero lol. I think as long as you’re proficient and know your zero and how it is at different distance it doesn’t matter. My hd at is zeroed at 25 yds. I don’t see my self needing to take a lethal shot at over 75-100. If you figure out the poi shift post it up I’d like to know.


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  5. Michael Branson

    Michael Branson New Member

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    Has the temperature changed dramatically where you live from July to October? Temperature changes affect optics (not just EOTech, gasp!) and also how bullets perform due to air density. Combined together, 3.5 MOA isn't unheard of at all.
     
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  6. Tim

    Tim I am....an enchanter. Staff Member Benefactor Charter Life Member

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    Did you do this before or after discovering the zero was off? I've found with that style mount that even a small amount of re-tightening the caps will cause a shift in POI.
     
  7. Jayne

    Jayne Just here for the memes Charter Member Supporting Member

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    20-25 degrees maybe? I can guess environmental for longer ranges but such dramatic shifts at 25, 50 and 100 seem like a mechanical issue.
     
  8. Jayne

    Jayne Just here for the memes Charter Member Supporting Member

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    Did it after I got home.

    I'm going to (gently) bang the scope around a bit, transition drills and stuff, not like with a hammer, and see if it holds this new zero.

    The only thing I can think of is that between the last use and this one is when I applied the lock tight. I remember removing, applying and re-torquing one at a time after first setting them all in a cross pattern over 3 passes, but honestly can't remember exactly when I did that.
     
  9. Zbizzle911

    Zbizzle911 Charter Freedom Lover Charter Member Supporting Member

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    I’d bet that’s your culprit


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  10. Michael Branson

    Michael Branson New Member

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    Yeah, 20-25 degrees isn't enough to worry about. I used to live in Missouri where it would be 104 degrees in August, and then freezing by the end of September sometimes. That will jack up some equipment, let me tell ya!

    I think your plan of rezeroing and running some drills is sound. And hey, a fun way to, uh, check your equipment, right?

    Let us know how it pans out, and good luck.

    One thing I will say is, the Vortex guys are solid. Even though we are competitors with them, our companies have respect for each other. If something really is wrong with the optic, call them up and they'll make it right with you.