POI changing after bouncing on ATV

Discussion in 'Long Range Scopes and Other Optics' started by D.Camilleri, Nov 18, 2012.

  1. D.Camilleri

    D.Camilleri Well-Known Member

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    I have recently been seeing a change in point of impact on my 338 rum after bouncing around on the handle bar rack on my ATV. I have a Sightron S-III 8-32-56 with burris tactical rings and an egw heavy duty 20 moa picatinny rail. After bouncing around on some rough two tracks antelope hunting, my POI moved up 3 inches and right 7 inches at 100 yards. I found this out after two bad hits on antelope. When I got home, I removed the scope and checked my base mounting screws, all tight. I remounted the scope, went back to the range and resighted my gun. I had to move the scope down 3 MOA and Left 7 MOA. When I was done, it grouped perfect.

    My big question is: Should I trade out my 20 MOA base for a 0 MOA Base so that the scope adjustment will be more centered in the tube? Currently I only have about 6 MOA of down travel in my up down adjustment. The S-III has 100 MOA of adjustment, so I really don't need the 20 MOA base to get to my max distance of around 1300 yards. Could having the adjustment be so close to bottoming out comprimise the integrity of the adjustments? The tracking of this scope has been very good, but I went out to the range to double check the zero and I found that I was shooting 3 inches high again. This was after bouncing around on the ATV again and would explain why I over shot a muley a couple weeks ago.
     
  2. bruce_ventura

    bruce_ventura Well-Known Member

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    That seems like a lot of POA shift. 3-7 MOA is huge. The scope installation sounds solid. Sightron scopes have a better reputation than that. I would not invest in a 0 bias base - seems unlikely that the problem is too much elevation adjustment.

    Are you sure the turret knobs are not rotating? I'm also wondering if you have a stock issue.
     

  3. Broz

    Broz Well-Known Member

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    I have seen more than a few of the EGW bases fail in a few different ways. The last two were moving on large caliber rifles with heavy scopes. I would pull it and inspect for egged holes or wear signs from it moving.

    Jeff
     
  4. D.Camilleri

    D.Camilleri Well-Known Member

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    The base moving was my first thought, and that is why I removed the scope and checked the screws. They were all tight when checked with my torque wrench. I also looked for marks to show if the base had moved and couldn't see any. The POI hasn't changed at all from just shooting, but it has changed twice in as many atv trips and it makes me wonder if the pressures on the crosshairs aren't even with the turret being so close to the bottom of the range. I verified the adjustment on sightrons website and this scope is supposed to have 70 moa of elevation not 100 like I thought. I emailed sightrons customer service to see what they have to say about this condition. In the past when I have contacted them, they like to see the turrets starting close to the center of travel. I guess I will see what they say.
     
  5. D.Camilleri

    D.Camilleri Well-Known Member

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    My stock is a Bell and Carlson Alaskan with full length pillar bedding, the barrel is fully free floated. My turrets are all set to 0 at 100 yards and I usually hunt with the elevation set to 3 moa which puts me dead on at 300 and then I dial from there.
     
  6. J E Custom

    J E Custom Well-Known Member

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    In my opinion 20 moa bases are to much for the distance you intend to shoot. a 5 MOA or at most
    a 10 MOA should keep you close to center of the adjustments,

    When you get to the extreme limit of the retical adjustments problems are common.

    I like to site the rifle in at 100 or 200 yards, look at the ballistic table for your load and then, using the drop chart look at the elevation required to hit center target at 1000 yards (Or your target distance) and add a MOA base with the same (Or close) MOA . This will help keep the scope close to center.

    This should keep you within the scopes range of adjustments.

    This may solve your problem.

    J E CUSTOM
     
  7. D.Camilleri

    D.Camilleri Well-Known Member

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    This is why I am thinking of going back to a 0 moa base. I am only 6 moa from being bottomed out on elevation with the 20 moa base. With a 0 base, that should put me at 26 moa which gives me a max of 44 moa up. At 600 yards, I am only dialing up 10.4 moa which would put me right close to the middle of travel. I can't remember off hand how much elevation I need for 1250, but I think it was around 30 moa which should still be fine.
     
  8. bigngreen

    bigngreen Well-Known Member

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    We've been thrashing a 338 RUM with a Sightron SIII 6-24x50 in Burris tac rings on a BEDDED EGW 20 MOA base with zero changes. It's had some hellish rides but it's in a padded case in the back of a side by side and trucks. Been dead on for almost a year now with probably 400+ rounds. The only thing that makes it shoot odd is carbon fouling. I try to give the EGW bases all the help I can, I've some of the early ones crap out but the new HD rail bedded or flat glued down hasn't been and issue.
    If an optic moves zero just because it's on a 20 MOA base it's not a LR optic to begin with IMO, the most drastic movement I've seen from one of my rifles was a bedding job I did that was not stress free and would torque the action, one day it would be one place the next time move several inches and then one day it went all to hell while I was shooting and I finally found the culprit.
    I would think if the scope was moving that far it would work around more when firing it, but the way those handle bar racks hold a rifle they could work over the stock I would think. Strap it down in a padded case with a bungie and take it for a ride and see if it moves then.
     
  9. D.Camilleri

    D.Camilleri Well-Known Member

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    How much downward travel do you have in your elevation turret? I only have 6 moa of down travel when sighted at 100.
     
  10. bruce_ventura

    bruce_ventura Well-Known Member

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    It sounds as though the base isn't bedded. If not, I would bed the existing base first. While I'm at it, i would stick a ~.015 inch shim under the front base mount. That will get it back to 0-10 MOA bias.

    I put almost no torque on the base screws during epoxy cure. I tighten the screws just enough to squeeze the epoxy. As soon as I feel resistance, I back off 1/8 turn and let the epoxy set. I'm after zero stress in the base.

    What action do you have?
     
  11. bigngreen

    bigngreen Well-Known Member

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    I'll check tonight for sure but I think it was under a full rev.
     
  12. royinidaho

    royinidaho Writers Guild

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    Timely thread.

    My Zeiss when zeroed at 200 is 4 clicks up from the bottom of travel. It's quite handy.

    I haven't checked zero since it came back from an ATV trip where it spent plenty of time riding on the front rinky dink rifle holder. The butt was in the holder the front was resting/bouncing on the bipod.

    It'll be interesting to see how it held up. Also, its not the best of bases and rings.:rolleyes:
     
  13. D.Camilleri

    D.Camilleri Well-Known Member

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    I went out and shot twice today. First time was at lunch time, first shot perfect, dialed two moa and hit 2 inches high at 100. I followed up that shot with three more and they were all an inch higher and not a very tight group. I was shooting with my bipod and a rear bag. I thought about my shooting and went back after work and performed an experiment. First, I took a piece of paper and slid it down my barrel channel and it was clear, then with the bipod mounted, I placed a slight amount of pressure on top of the scope and the paper was pinched. I removed the bipod and shot two shots at 100 with the scope zeroed at 100. Two shots touching in the bullseye. Hmmmm, time to clearance the stock in the barrel channel. I think I have found the problem. I am still going to put on the 0 cant base when it shows up so that I can at least see the bottom hash marks on the elevation turrret of the scopel
     
  14. bigngreen

    bigngreen Well-Known Member

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    Sounds like your dial in on the issue! We just under a rev from the bottom, my buddy didn't want to check the exact number because he was afraid it would put some bad juju on it or something but it's a little more than what your at.