Ensuring scope reticle is perpendicular to action [avoids cant POI error at extended ranges]

No it wouldn't, because the barrel would no longer be angled upwards relative to the line of sight through the scope. It would be angled to the right relative to the line of sight through the scope, so there would be no parabolic curve. The bullet would bear off to the right, and fall.

You are envisaging a rotation around the axis of the scope.

@gixer1 is envisaging a rotation around the axis of the bore.

Alan

p.s. I think...
 
In a practical situation, in the field, in order to make a correction for cant, would you not automatically rotate on the axis of the line of sight through the scope, not the axis of the the bore? So while Gixer's points may be correct in theory they aren't relevant in the field?
 
In a practical situation, in the field, in order to make a correction for cant, would you not automatically rotate on the axis of the line of sight through the scope, not the axis of the the bore? So while Gixer's points may be correct in theory they aren't relevant in the field?

I think in the field none of this will really make much difference anyway! 😂😂
 
I watched a Randy Newberg YT clip last night in which he explains how a non-verticle reticle causes cant related POI shift at distances other than the zero'd range. His tutorial has easy to follow graphics and he shows how he achieves action-to-scope truing using some clever optical device. But I got to thinking I could probably MacGyver a way to achieve the same outcome...



As it happens, the scope on my 30.06 was slightly off and needed fettling so I had a candidate for the project.

First up I lashed a visually straight bar onto the flat top edge of of the receiver action and ensured that there were no slivers of light where the action and reference bar married View attachment 175280. Next I suspended a plumbline 2m in front of the rifle and manouvered the rifle bench to face that. Scope was set on lowest zoom, reticle and plumbline almost overlapping in sight picture. Then I canted the stock until declared level View attachment 175281. Reviewing the sight image now, the plumbline and reticle were not in lockstep [see next post for before and after sight pics] and a spirit level placed on the top turret further confirmed cant View attachment 175283. So I loosened scope clamping screws and rotated scope perhaps 3 degrees anti-clockwise. I double checked that rifle action was still level and checked both spirit level and plumbline sight picture before tightening scope screws. One more final check after that View attachment 175293.


So the before and after sight images looked like...

You're assuming that the turret cap is true to the horizontal cross hair when laying the spirit level on it's head ;)
 
You are envisaging a rotation around the axis of the scope.
@gixer1 is envisaging a rotation around the axis of the bore.

Sure. But that is not how we shoot. We look at targets through scopes.

Even if gixer1 is envisaging a rotation about the bore rather than the sightline, the POA line and POI arc of a zero'd rifle never intersect if rifle is fired subsequently lying on its side.

So it cannot be true to say of a rifle fired whilst lain on its side that:

you could in theory fire a rifle upside down or 90° to one side or the other and as long as the scope is plumb to the target and mounted to the rifle securely wouldn’t matter.
 
Last edited:
+1 for the spuhr mount. takes about 5 seconds if that. if you 'just eye' everything in youll have a few grey areas. i try not to just eye my cbto measurement or how much powder i use!
 
+1 for the spuhr mount. takes about 5 seconds if that. if you 'just eye' everything in youll have a few grey areas. i try not to just eye my cbto measurement or how much powder i use!

After the scope set up, a built in level to check you are holding the mount reasonably plumb or at least reasonably consistent to the bubble when shooting would not be a disadvantage of course.

But don’t knock “just eye”. It is the most precise and sensitive tool we all have. You use it to read your powder scale and the caliper or micrometer for your CBTO.

If "just eye" can read a vernier scale accurately why is it incapable of aligning the reticule to the bore centre accurately?

You rely on your eye to read a bubble level which is much less precise. Unless you are paying thousands for your level it will probably be built to a price rather than a specification. Very easy to view slightly off square and have parallax misreadings even if it is more sensitive than a builders level.

The guy in the OP video introduced four bubbles, each with its own accuracy tolerance in relation to its datum surface which he then trusted his eye to read. At no time did he relate the scope rotation directly to the bore centre, just to a grid set by a bubble on a flat bit of the action which was also set to a bubble. This seems counter productive to me...

Why not “just eye” the relationship of reticule and bore directly, with out the introduction of multiple devices with their manufacturing tolerance and the associated cumulative errors?

If you set your scope by “just eye” and check it with a couple of torches or sighting through both bore and scope to a plumb line you cut out any chance of cumulative errors.

Alan
 
  • Like
Reactions: VSS
After the scope set up, a built in level to check you are holding the mount reasonably plumb or at least reasonably consistent to the bubble when shooting would not be a disadvantage of course.

But don’t knock “just eye”. It is the most precise and sensitive tool we all have. You use it to read your powder scale and the caliper or micrometer for your CBTO.

If "just eye" can read a vernier scale accurately why is it incapable of aligning the reticule to the bore centre accurately?

You rely on your eye to read a bubble level which is much less precise. Unless you are paying thousands for your level it will probably be built to a price rather than a specification. Very easy to view slightly off square and have parallax misreadings even if it is more sensitive than a builders level.

The guy in the OP video introduced four bubbles, each with its own accuracy tolerance in relation to its datum surface which he then trusted his eye to read. At no time did he relate the scope rotation directly to the bore centre, just to a grid set by a bubble on a flat bit of the action which was also set to a bubble. This seems counter productive to me...

Why not “just eye” the relationship of reticule and bore directly, with out the introduction of multiple devices with their manufacturing tolerance and the associated cumulative errors?

If you set your scope by “just eye” and check it with a couple of torches or sighting through both bore and scope to a plumb line you cut out any chance of cumulative errors.

Alan


Killjoy







;):)
 
Perhaps a good summary to this thread:

Why not “just eye” the relationship of reticule and bore directly

because

I worked in a gunshop and three of us used to fit scopes for customers. We all had a different result if we aligned them by eye


Pretty much all of us know that shooting at ranges around our zero distance garantee we will make ethical shots.

The OP was about setting up a rifle for more predictible long range ballistics where errors introduced in optical alignment will have a magnified effect. The takeaway from all the contributions must be that you need to practise at all distances at which you intend to shoot [don't just extrapolate close zero data].

Anyone got a 500m range I can borrow in North Devon? :)
 
You are right alantoo, i just eye the digital scales and verniers. Also depends on your rifle. A custom action with machined in rail will have alot less tolerance than bolting a rail to an old parker hale. Everone does different things with their rifles on the workbench and in the field. If its working for you no need to change
 
Hi it is important to ensure the scope is vertical to the action. As I have posted years ago I use a piece of plate glass which is placed across the bottom half of the mount and use that to ensure the action is level and then align the reticule using a plumb line which is hanging 20 yd down the garden.

Some issues.
Mounts are misalinged as glass should rest equally on all the parts of the mount. Sometimes may have to reverse or swap them arround.

Dont use the screw cap method as there is published date and its on here that even in the top manufacteurs that the reticule can be misalinged up to 5 degress from vertical.

Some scopes have internal bubbles eg Konus, Pecar, these might not be level.

Whilst aligning the scope its also handy to fit a rail or scope mointed spirit level. It doesnt matter where it reads just note where the bubble reads when the action is level.

Where levels are handy other than long range shooting is when you are shooting acros down or up a sloping bank esp if it is steep. If you use a level you will be surprised to see how much you cant the rifle. Source of many missed bunnies when shooting rabbits on steeply sloping banks in Wales.

You only have to do it once so get it correct the first time you fit a scope.

D
 
Back
Top