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Sight picture question

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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: February/02/2007 at 08:36
stumpy View Drop Down
Optics GrassHopper
Optics GrassHopper

Joined: March/14/2006
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This question may of be covered before but I could not find a good answer. Its hard to explain my question but here goes...If you have your scope zeroed for your rifle and the scope WE adjustments happens to be perfectly in the center of thier adjustment range any error/canting in your hold of the rifle (sight picture) would have no effect on the POI except accounting for bullet drop...however if your scope is zeroed for your rifle and as is common the scope WE adjustments are offset from the center of the adjustment range of the scope then any error in your sight picture (canting) would be more exgagerated resulting in a more open shot group....Of course the scope is mounted some distance above the bore thus in the adjustment perfectly centered sitution the worst cause would be a group which would equal the diameter of this scope to bore distance but how much effect or difference does the reticle adjustment away from center have on your POI if canting occurs .. is the effect a function of the actual distance the retitcle actually moves inside the scope as compared to the scope to bore for example 1/64" actual reticle movement plus the scope to bore distance or is this a function of the percieved distance at the target for example 1/4" per click?
Clear as mud

Thanks in advance

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: February/02/2007 at 09:23
Urimaginaryfrnd View Drop Down
Resident Redneck

Joined: June/20/2005
Location: Iowa
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Koshkin  will gladly tell us no doubt.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: February/02/2007 at 22:08
koshkin View Drop Down
Dark Lord of Optics

Joined: June/15/2004
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First of all, as far as group size is concerned it really does not matter whether the rifle/scope is canted or not, as long as it is held the same way for every shot.

Now, POI will certainly depend significantly on how you hold your rifle.  What you posted about the scope having to be in the center of W/E adjustment range is not strictly speaking correct.  The center of the W/E adjustment range is something called the Mechanical Center (or sometimes referred to as Mechanical Axis).  There is also an Optical center (or Axis) which does not always coincide with the Mechanical Center.  If your scope is in its Opical Center, then canting the rifle will cause a comparatively small difference in POI. 

Here is a description of the difference between optical and mechnical centers:

With the scope in v-blocks, align the crosshairs with a distant point, then turn the scope in the v-blocks.  If the crosshairs wobble, if they move off the distant point, then adjust them until they do not move relative to the distant point when the scope is rotated around its axis.  That is what most people mounting scopes refer to as the "optical center".  That is, the crosshairs are aligned with the centerline of the optics.

Keeping track of the dials, turn the elevation knob until it stops.  Then turn it the other way until it stops.  Set it halfway between those stopping points.  Do the same with the windage knob.  That is what most people mounting scopes refer to as the "mechanical center".  That is, the adjustments are in the center of their  mechanical range.

On a good scope, the twocenter will more or less coincide.  Any significant misalignment of the ""center"s can indicate a problem with the scope.

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