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Old scope - New problem

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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/12/2015 at 14:11
Dawgdad View Drop Down
Optics GrassHopper
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I have a Mueller Sport Dot 3-10X that I have had mounted on a WOA varmint 1:7 AR with a RRA - PEPR type mount for many years. I have always checked the zero at 100 yards and have never noticed any significant issues with POI shifts when changing magnification at this distance. Normal load is a 55 V-max with Varget. I set it for 250 yard PBZ and have taken quite a few furbearers with it. I love the illuminated sport dot reticle and the glass clarity and all other functions of the scope turrets for adjustments. Usually a once a year set it and forget it type deal.

Now the problem -  I had a chance to shoot the rifle at 600 yards with Sierra 77TMK's over the weekend.  ( I compete in Highpower with service rifle and normally shoot 600 with iron sights at a Master rating  95%)  I added about 20 MOA of elevation and was in the X ring pretty quickly and kept it in the ten ring for a half dozen shots.  The 1 moa dot just covered the 1 moa X ring as expected.

I then dialed down to 4x as a proposed rule change would allow 4x optics in service rifle. The center dot now covers the entire 10 ring (2 MOA) as I thought it would at about half the magnification.

Next shot was 18" off to the 8:00 position (wide 8 on the MR target). Hmmm.... shoot again. shot the spotter in the same place.... added 2 moa up and 2moa right..... shoot a 10.  Ran out of ammo before I could get a chance to turn the power back to 10x and see if it was still off.


My question is was this POI shift most likely due to  the fact that I had cranked up 20 MOA from where the crosshairs normally are more centered in the optical system or has it always been off a bit but I just never noticed it at closer ranges?



Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/13/2015 at 08:19
3_tens View Drop Down
Optics Jedi Knight
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First thought is that Wind is not your friend. It is not as noticeable at 100 yards. Things get tricky past 400 yards +. Wind deviation is on a curve rather than an angular deviation.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/13/2015 at 08:20
urbaneruralite View Drop Down
Optics Apprentice
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Your problem is most likely that the scope adjustments are not dependable or accurate enough for repeatable vertical adjustment.  Adjustment of the zoom control probably moved things around enough to cause the windage shift.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/13/2015 at 11:10
Dawgdad View Drop Down
Optics GrassHopper
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Thanks for the input.

3 tens - If I had not just completed a match firing shooting a 191/200 on the same target I might that agree that wind is not your friend but I had the dope down for that condition pretty good... 3/4 moa left was all the wind that was there.  I shoot a lot at 600 just not with a scope.


This particular scope has 1/8 MOA clicks so it would have to jump 16 clicks in two directions...

Not saying it isn't possible though.



Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/13/2015 at 14:54
Dawgdad View Drop Down
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I talked to an guy with a degree in laser optics who had a job offer to work for Leupold coming out of college a decade ago. He suggests that the front lens in the variable power system was shifting laterally in the tube as it is moving to change magnification causing the image to slide relative to the fixed cross hairs.


Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/13/2015 at 20:33
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Optics Jedi Knight
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Originally posted by Dawgdad Dawgdad wrote:

Thanks for the input.

3 tens - If I had not just completed a match firing shooting a 191/200 on the same target I might that agree that wind is not your friend but I had the dope down for that condition pretty good... 3/4 moa left was all the wind that was there.  I shoot a lot at 600 just not with a scope.


This particular scope has 1/8 MOA clicks so it would have to jump 16 clicks in two directions...

Not saying it isn't possible though.



I have seen a lot of scopes run a circle when adjusting the objective for parallax. Just not as many adjusting the power ring. Any slack in tolerances between moving parts will create some alignment errors. This can be observed to some degree even in what is considered the better scopes.
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