Eye Relief and POI

[GatorCountry]

Would adjusting the scope in its mounts (moving it forward) affect POI?

I spent time this weekend working out a new DOPE sheet for my rifle.  By Sunday's session I started thinking that I hadn't set the rifle up with enough eye relief when in a seated HFT position.  Not really wanting to lose all that work is influencing my decision making process now.  I won't have time to experiment shooting until this next weekend, so thought I'd ask the experts here.

In theory, I'd think if the move forward of the scope is in perfect alignment with the bore, there would be no effect.  Scope cant correction and the rest of the setup remaining the same.  In practice, that perfect one dimensional movement may be hard to achieve.

Also, not sure if this is a general air gun question or would have been better in the Optics gate.  If needed, moderators please move.

[DevilsLuck]

I would guess yes...

[bandg]

One would think that it should not change but seems like it usually does. 

[oldnamvet]

It seems that when you loosen the mounts in any manner and then retighten, the POI will change --- perhaps not much but it will be noticeable. At least that has been my experience.

[lizzie]

My experience is that it definitely impacts POI.
I have had shooting sessions when I'd notice a show gradual shift in POI, then upon inspection of the gun, noticed the scope had migrated. Repeating that process reinforced my findings.

[bandg]

Notice specifically what OP originally asked.  He noted moving the scope in the mounts, not moving the mounts on the gun.   Moving the mounts on the base/dovetail would certainly seem to introduce more inherent change than just moving the scope in the mount.  But seems most if not all of us have noted that scope moving (or being moved) in the mount does produce change in POI.  My question is WHY this would happen.  The mount just establishes an angle between the body of the scope and the bore.  Moving the scope fore and aft in the mounts theoretically would not change that angle at all.  So WHY does the POI change.  Is it an optical/visual thing, is it change in pressure on the scope body causing a mechanical change internally, is it misalignment of the two mounts, or is it something else?  Any ideas?

[Nvreloader]

I have found that with LL,
when I remove the scope, (at least 25-30 times now), rings stay attached in the same spot on the scope,
then reattach the scope to the action, within .001" measured of the original spot and using the same torque spec's etc..

I have found a POI changes, regardless how careful I am.   
Nature of the beast?

Tia,
Don

[lizzie]

Notice specifically what OP originally asked.  He noted moving the scope in the mounts, not moving the mounts on the gun.   Moving the mounts on the base/dovetail would certainly seem to introduce more inherent change than just moving the scope in the mount.  But seems most if not all of us have noted that scope moving (or being moved) in the mount does produce change in POI.  My question is WHY this would happen.  The mount just establishes an angle between the body of the scope and the bore.  Moving the scope fore and aft in the mounts theoretically would not change that angle at all.  So WHY does the POI change.  Is it an optical/visual thing, is it change in pressure on the scope body causing a mechanical change internally, is it misalignment of the two mounts, or is it something else?  Any ideas?

The angle of the scope in relation to the gun should not change, but the location of the scope itself does change in relation to the gun (and rarely is the gun itself held at a perfectly level position). That would impact POI. It isn't much, but my experience is that movement of the scope (twithout movement of the rings) does change the POI. I have had it happen several times. In my case, it gradually moves the POI upward and slightly to the right

[Novagun]

It seems that anything you do to an air rifle changes the poi. If nothing else in this case the shifting of the scope in the rings will alter the centre of gravity of the rifle. That will make a difference to the recoil and the barrel flip and scope wobble on discharge. Even holding your head back or forwards to get a good site picture makes a difference.
Makes one wonder at why we persevere. At least if gives us something to talk about; now a shot gun -- who knows.

[dan_house]

dope sheet data shouldnt change.

Move the scope in anyway shape or form and youll need to rezero. It may not be much but itw ill move

[GatorCountry]

Notice specifically what OP originally asked.  He noted moving the scope in the mounts, not moving the mounts on the gun.   Moving the mounts on the base/dovetail would certainly seem to introduce more inherent change than just moving the scope in the mount.  But seems most if not all of us have noted that scope moving (or being moved) in the mount does produce change in POI.  My question is WHY this would happen.  The mount just establishes an angle between the body of the scope and the bore.  Moving the scope fore and aft in the mounts theoretically would not change that angle at all.  So WHY does the POI change.  Is it an optical/visual thing, is it change in pressure on the scope body causing a mechanical change internally, is it misalignment of the two mounts, or is it something else?  Any ideas?

Precisely the thinking behind the original question.  In my experience, changing anything related to the scope seems to affect POI, but just moving the scope in the mounts shouldn't.  Not sure that I've ever tested this single change though, which led to the post.  However a scope that isn't torqued correctly and moves as a unit (scope and rings) will change POI.  Either way, it implies the second question - why?  Might be multiple factors, unique to the rifle or shooter.  Change in eye position with parallax not perfectly compensated, (slight) misalignment of rings making an undetected change in angle, etc.

[GatorCountry]

The angle of the scope in relation to the gun should not change, but the location of the scope itself does change in relation to the gun (and rarely is the gun itself held at a perfectly level position). That would impact POI. It isn't much, but my experience is that movement of the scope (twithout movement of the rings) does change the POI. I have had it happen several times. In my case, it gradually moves the POI upward and slightly to the right

We might be making the same observation...moving the scope forward not change the relative position to the rifle, but would change it's position in the geometry of the line of sight through the scope, relative to the curved trajectory of the pellet's path.  I'm not the mathematician to figure that out in percentage terms, but I'd have to think the variance was pretty small, even if the scope was moved a cm or two.  Then again, small mm errors at the muzzle translate into big differences down range.
 

[GatorCountry]

dope sheet data shouldnt change.

Move the scope in anyway shape or form and youll need to rezero. It may not be much but itw ill move

This is what I'm really hoping for and seems to make sense to me.  Reset the relief, check cant, re-zero at the same yardage - use the range card without change.  The holdovers will remain relative to the (corrected) zero.

[nervoustrigger]

Simply removing and reattaching a scope via the rail will retain the zero quite well in my experience.  However, loosening the scope caps will usually shift the POI very slightly, even given reasonable care not to rotate it or shift it fore or aft, and torquing to the same value.

Intuitively that's pretty much what I would expect.  The clamping interface to the rail is metal on metal with (usually) one side of the jaw being fixed.  There's no "give" to anything.  Conversely, the scope tube will deform slightly in ways that may not be entirely repeatable given the typical 4 screws per cap.  Shifting the scope fore or aft in the rings will almost surely result in a different degree of deformation because the rings will be clamping on a different spot where the tube is more or less rigid due to proximity to the to the bells on either end or the turret body in the middle.

[GLPalinkas]

dope sheet data shouldnt change.

Move the scope in anyway shape or form and youll need to rezero. It may not be much but itw ill move

I agree 100%.... POI will change. Scope dope will not.

[Relentless Holiday]

Another yes.  Those quick detach mounts are fine for tactical short range Minute of chest cavity stuff. But like others said even changing the tourque on the rings of a mounted scope will change stuff when you are really dialed in well.

[lefteyeshot]

Eye relief has to be right. If you have to move the scope to achieve that, POI will change so you'll have to re-level and re-zero the scope.

[Relentless Holiday]

Had a thought on this.  Too much maybe... hehehe

Think of a picture in a math book of an angle.  The POI is the actual point of the two lines meeting, just pretend your pellet goes straight with me on this. Also think of the length of the two lines being constant.  They ARE the same relatively till you move something, but if you move either of the lines the angle needs to change (your aim) or the lines can't meet any more.  You are moving the end point of one of the lines so you need to resight for the different angle. And  that's only one one plane.  If you havn't mounted the scope 100% aligned with the barrel VS the reticle you have a pyramid point with three lines involved and not just a angle to account for. 

Yeah, It's changes everything....

[WhatUPSbox?]

Let's say your rings are 3" apart. Less that 0.001" relative radial change of one to the other gives you a minute of arc change in where the scope is pointing. So the tolerances of the parts, their stiffness when clamped, torque values, etc. all roll up into that small number. On top of that you are bending the scope slightly differently as well.