POI movement when you alter scope position

[DaveX]

HI. Has anyone had any experience on the point of impact changing when you alter the scope position on a 40moa rail. That is moving the scope position either forward or back on the notches within the rail.

[bully_eye]

Yes. Height above bore changes slightly which should effect your elevation. I am talking small amounts here.

Michael

[bruce moulds]

possibly, but i think not.
it is all about angles, so if poi changes something is under differing ammounts of stress that should not be there.
keep safe,
bruce.

[bainp]

Hi DaveX
Any target rifle shooter will tell you that if you raise your rear sight by 50 thousandths of an inch (ie. 5 minutes of angle) then it will raise the bullet impact by 5 inches per 100 yards, BUT, if you raise both your front and rear sight by 50 thou then it makes no difference on the target. This is not entirely true, it makes 50 thou difference on target, but I challenge any target rifle shooter to tell me that their group has shifted by 50 thou at any target rifle distance!!!!
Bruce is 100% correct, it is all about angles.
If we now go to moving the position of a scope on the rail. In your example, you mentioned a 40 minute rail. If you move the scope from the very front of the rail to the very back of the rail, the angles are unchanged but the scope will have increased in height above the bore by say 50 or 60 thou. The group will alter in position at all ranges by 50 or 60 thou. I challenge any shooter, including scope shooters, to tell me that their group has shifted in position by 60 thou, unless of course he/she is shooting at point blank range.
Cheers - Philip

[bruce moulds]

philip,
we are both right.
i came back to say i was wrong, but you described it correctly.
keep safe,
bruce.

[AlanF]

There are a couple of ways you may get detectable POI change when moving between slots. If a rail has imperfections e.g. is warped slightly, or has two parts which don't align perfectly, or it is machined imperfectly, then you will get POI changes. Also if the base of your rings are not perfectly parallel with the plane of the rail, then the scope may clamp back on differently, even into the same slots of the rail.

[RJNEILSEN]

Any poorly fitted rail will cause a poi shift when moving the scope. Again as already stated, its all triangles and trigonometry. If the rail is on the piss, then moving the scope has to alter poi.

Cheers,
Ryan.

[Gyro]

"Do the maths lads" : with the scopes ring centres at say 6" and the target at 100 yards/3600".

3600 ÷ 6 = 600. That's a 600x error ratio.

So just 0.001" ( one thousanths of an inch ) relative movement between scope rings spaced 6" apart will give you 600x that much POI movement at 100 yards. And 600 thousanths is just over 15mm.

It's the same thinking when you do similar maths for how a gun behaves during the two milliseconds of barrel time before the bullet exits ........ it's another subject but same principle.

I would totally expect the POI to shift with any scope setup change on any rifle for a whole bunch of simple reasons !!

[bainp]

Hey Gyro
I did do the maths and my maths comes to the same conclusion as your maths, ie. 0.6 of an inch movement of POI at 100 yards for each 0.001 inch of runout on the scope base.
But I thought everyone would check their scope base for trueness before installing it on their rifle. I certainly do. I also bed all my scope bases to the action, to ensure that there is no bend in the scope base as I tighten it to the action. Just to be certain to be certain. Most of us check so many other things to the most pedantic degree, why wouldn't you check for trueness of your scope base?
The amount of change of POI of your example of 0.6 inches per 100 yards is well within what changes I would expect from day to day caused by environmental/weather conditions on my local range.
I think DaveX needs to check for runout or a bend in his scope base if he is having noticeable changes to POI when moving the scope back and forth along the scope base.
Regards Philip

[Wal86]

There are so many elements that could be causing this POI movement..
This scope like many hasnt been fitted correctly and there is differing amounts of stress causing this shift...
(Without going into to much detail)
Guaranteed scope rings have not been lapped and bedded, base was never checked and it has a hog in it..
If base does have a hog, now your scope does..
I will also assume that alot of horizontal adjustment has been taken up on this particular scope to get it in line.
The higher the magnification the greater these stresses impact on a scopes performance.
Cheers
Alan

[MR.Sako]

Tapered rails are tapered reletive to the bore so any movement, fowards or backwards results i poi shift

[bainp]

DaveX you are nearly there

[MR.Sako]

Very highly technical explanation

[bainp]

Yeh Yeh
I give in

[bainp]

Who are the agents for those scopes??? I must try to get onto one of those.