Changing POI

Discussion in 'Long Range Scopes and Other Optics' started by lisagrantb, May 20, 2013.

  1. lisagrantb

    lisagrantb Well-Known Member

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    I’m trying to solve a changing POI and wondering if anybody has had this happen to them. I’ve tried this with two different guns and the results are the same. I put the gun in a barrel vise and wrap my hands around the scopes objective bell and the barrel or the barrel and stock and squeeze about as hard as a firm handshake and release. This applies downward pressure on the scope, if I look through the scope at the target the cross hairs moved about 1.25” at 100 yards. I can then use two fingers and wedge between the bell and barrel, this applies upward pressure on the objective bell then release and look through the scope again the cross hares will move back. I have verified this at the range. It shoots 2 nice .4” groups 1.25” apart all vertical. One gun is a defiance action with Seekins base and rings the other is a 700 with Nightforce rings and bass both have NXS scopes on them. I am very particular when mounting the base and scope and have checked and rechecked everything. I also put a dial indicator on the base to see if it moves relative to the action and it's rock solid. any thoughts on this

    Sorry the photo came out rotated 90 deg. the two groups are actually top and bottom
     

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  2. MudRunner2005

    MudRunner2005 Well-Known Member

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    Had the POI change a couple times on a couple different guns. For mine it was crosshairs were broke, or something was loose in the scope mounting.

    I have no idea what could be going on with yours. Do you have your forward scope ring far enough forward to properly support the big objective bell with a large magnum, so it isn't vibrating too much? B/c that can sometimes cause what you are describing.
     
  3. lisagrantb

    lisagrantb Well-Known Member

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    It's a heavy 14 # gun shooting a 260 Remington so it's not a magnum. I did play with the front ring placement moving it as far forward and backwards as I could and the change was always there. I only used the barrel vise to verify the movement when I did this. The ring was half way between the turret and the objective bell when I did the range test.

    I think I have always had this problem with the Defiance but I installed a break and it made the problem really worse. Before the Break my groups would show signs of vertical stringing about .75" about 50% of the time and other times it would be nice .3-.4" groups. I always thought it was my technique.
     
  4. X-man

    X-man Well-Known Member

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    So you only get changing POI when you push or pull on the scope?
    I don't follow your question?
     
  5. MudRunner2005

    MudRunner2005 Well-Known Member

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    Make sure your action screws are torqued properly, and make sure your barrel is 100% free-floated and that nothing is touching.

    Could also need to bed your action to the stock.

    Sounds to me like barrel harmonics if the muzzle brake made it worse...
     
  6. lisagrantb

    lisagrantb Well-Known Member

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    The POI change happens all the time but its hard to know when and how much when you are shooting in the real world, the recoil changes it, if I pick the gun up by the scope or put pressure on it when getting into a shooting position. The range test I did only changed one thing. I didn't move the gun off its rest all I would do is put upward pressure on the scope and shoot one shot then put downward pressure then shoot one shot, I alternately did this to get the two groups in the photo.
     
  7. lisagrantb

    lisagrantb Well-Known Member

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    The barrel is floated and the action is bedded. I've tried really hard to duplicate the problem without touching the scope and I cant, i've tried prizing and pulling everything without touching the scope and it doesn't have ay affect until i mess with the scope. I removed the break and ran the test and I get the same results.
     
  8. X-man

    X-man Well-Known Member

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    You have a lot of variables going on and can't see the forest for the trees.
    When I see this it leads me to believe this is most likely a shooter problem.

    Have you tried putting the rifle in a rest and eliminating any shooter error?
     
  9. lisagrantb

    lisagrantb Well-Known Member

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    I'm not disagreeing with you but when I put the gun in a vise and watch the crosshairs move on the target, that isn't shooter error and the range test repeats exactly what the vise test shows.
     
  10. MudRunner2005

    MudRunner2005 Well-Known Member

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    If the crosshairs change on their own, without you touching the scope, then you have internal damage to the scope itself...
     
  11. X-man

    X-man Well-Known Member

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    While it is possible the Nightforce has a problem...the odds most definitely are not in your favor.

    I don't doubt that you can change POI. You can't put pressure on a full floating barrel or scope/receiver and not expect a change.

    Shoot the rifle in a cradle/rest or ask an experienced shooter to test the rifle for you. Once you eliminate the possibility of shooter error you can move ahead.

    Your drifting point of aim could easily be attributed to trigger operation or rifle/body position errors.
     
  12. lisagrantb

    lisagrantb Well-Known Member

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    I can get my other accurate rifles and shoot .5 or less and mostly less around .3-.4" 5 shot groups all day long. The group in the photo is typical and it happens the same every time. every time I put upward pressure on the scope it shoots low and every time I put down pressure it shoots high not most of the time but every time. If it were me it wouldn't repeat that every time. The group in the photo would be even smaller if i had compensated for the 10-15 MPH full value left to right wind, I just held center and shot. I don't think I could shoot 2 .3" groups by being a bad shooter.
     
  13. lisagrantb

    lisagrantb Well-Known Member

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    "I don't doubt that you can change POI. You can't put pressure on a full floating barrel or scope/receiver and not expect a change."

    I guess that is my question. Can you and if so is it normal to change the POI by putting pressure on the barrel, receiver or scope. I'm not talking jumping up and down on the thing but something like picking up the gun by the ocular bell or getting up from prone position and putting pressure on the gun while doing it
     
  14. X-man

    X-man Well-Known Member

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    You should have said that earlier.
    Put another scope on the rifle.