Scope leveling

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Skip-in-WV

Skip-in-WV

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Apr 5, 2015
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Morgantown, WV, United States
I used a bubble level (actually two) mounted on opposite sides of the scope rail and a plumb bob string to level the scope. When I go out to shoot in my FT position sitting on a bum bag the reticle looks off about 5 degrees to the left when the rifle is bubble level. It must be because I tilt my head. My question is what effect, if any, would it have on the POI at 50 yards (with a 25 yard zero)) if I tilt the scope 5 degrees so it looks level to my eye? 
 
Assuming a JSB 10.3gr at 900fps (18.5fpe), you are looking at a horizontal POI shift of about 0.55in. More if it's moving slower, and slightly more if it's a pellet with a poorer BC.

If you want to figure it yourself, use something like Chairgun to determine the drop...it's one of the default columns in the Ballistics Table on the main page.

Then calculate the horizontal shift as SIN(cant angle) * drop
 
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I used a bubble level (actually two) mounted on opposite sides of the scope rail and a plumb bob string to level the scope. When I go out to shoot in my FT position sitting on a bum bag the reticle looks off about 5 degrees to the left when the rifle is bubble level. It must be because I tilt my head. My question is what effect, if any, would it have on the POI at 50 yards (with a 25 yard zero)) if I tilt the scope 5 degrees so it looks level to my eye?
Click to expand...

If you suffer from astigmatism you may see a canted image. This will not be obvious during normal conditions but will be evident looking through a scope reticle. I have that in my right (shooting) eye. I see the vertical crosshair canted but not the horizontal crosshair.

My left eye is normal. If I switch to left handed, the crosshairs appear normal.



The only thing one can do if they have this condition is just concentrate on the horizontal crosshair being level. It can be corrected with the correct eyeglass prescription.

I do not have issues with a shifting (horizontal) POI at longer yardages.
 
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Note, regarding the levels and plumb bob approach for installing the scope, that will work reasonably well most of the time but it assumes everything about the gun and scope and scope mounts are mechanically perfect. Modern machining methods are generally pretty good but it's a long list of assumptions, some of which are:

1. The barrel is perfectly straight, and
2. the bore is centered within the barrel, and
3. the scope rail is precisely parallel to the receiver's mortise, and
4. the barrel-to-receiver interface has no lateral bias (a typical O-ringed barrel tenon secured with a grub screw through the top of the receiver will have some bias), and
5. the scope mounts are precisely centered above the rail, and
6. the reticle is clocked perfectly to the scope body/turrets, and
7. the barrel band (if present) isn't biasing the muzzle, and 
8. the selected flat feature on the gun is perfectly perpendicular to all of the preceding items.

So you may want to try this to check the result...set the scope's magnification to max and AO to the shortest distance. Set up a rest about half that distance from a mirror and look through the scope into the mirror at your reflection. Confirm the vertical bar of the reticle simultaneously bisects both the muzzle and the objective bell. If not, you need to rotate the scope in the mounts so it does.

It's helpful to tape a light-colored dot to the muzzle to make it easier to see in the reflection. A sliver from a Post-it note works well, a dot from a hole punch, etc. 

Then fine tune your scope-mounted level as needed. That is, view a plumb line through the scope and align the reticle to it, then secure the spirit level so it shows level.

From this point forward you can trust that your scope and barrel are aligned properly to each other, and that your level will help you keep them oriented to the force of gravity.
 
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Indeed, that's quite useful if you will be dialing the turrets. If using mildots, the "tall target test" does not apply because you would be overlaying a potential erector tube tracking error with a potential scope alignment error. Still of course good to verify with actual shooting in either case. I just like to start with a scope alignment that is as confident as I can reasonably make it before heading outside because the wind is so unforgiving with pellets compared to centerfire rounds.
 
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Indeed, that's quite useful if you will be dialing the turrets. If using mildots, the "tall target test" does not apply because you would be overlaying a potential erector tube tracking error with a potential scope alignment error. Still of course good to verify with actual shooting in either case. I just like to start with a scope alignment that is as confident as I can reasonably make it before heading outside because the wind is so unforgiving with pellets compared to centerfire rounds.
Click to expand...

So, you are saying the "tall target" test couldn't tell a strictly mil-dot shooter whether his scope was level or not?
 
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And so, you now eliminate those who click-to from the discussion by "pivoting" to mil-dot? Still seems like a pretty egocentric point of view to me, since apparently many people use both or either methods. When including click-to benefits one's view, it is included. At other times, it is excluded. The "tall target" method works to insure BOTH that scope is LEVEL (which a "mirror method" can do) and that it TRACKS reliably along the vertical (which a mirror method "cannot" do). Seems both might be relevant to SOME SHOOTERS, but a quick and easy shortcut (the mirror method) that gets one halfway there CAN still be useful. Seems I recall some mention of absolute accuracy in another discussion? All relative.

Would you care to CLEARLY describe the "modified" version of the tall target method that WOULD work with mil-dot shooting?
 
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And so, you now eliminate those who click-to from the discussion by "pivoting" to mil-dot?
Click to expand...

Oh no, I did the opposite of what you claim. You say I eliminate (exclude?) those who "click-to". What I did was affirm the tall target test will work well for those who "click-to". How you twist it into the opposite is a another malicious attempt to mischaracterize me and it is plainly obvious to everyone reading it.

The "tall target" method works to insure BOTH that scope is LEVEL and that it TRACKS reliably along the vertical.
Click to expand...

Yes it does...if you are dialing the turrets. It does not if you are using mildots. For example, you could have the scope installed canted and the turret could move the reticle at a complementary angle, making it appear that both are working perfectly. However if you chose not to dial and instead use the subtensions on the reticle, the POI would be off.

Would you care to CLEARLY describe the "modified" version of the tall target method that WOULD work with mil-dot shooting?
Click to expand...

No I would not.


 
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And so, you now eliminate those who click-to from the discussion by "pivoting" to mil-dot?
Click to expand...

Oh no, I did the opposite of what you claim. You say I eliminate (exclude?) those who "click-to". What I did was affirm the tall target test will work well for those who "click-to". How you twist it into the opposite is a another malicious attempt to mischaracterize me and it is plainly obvious to everyone reading it.

The "tall target" method works to insure BOTH that scope is LEVEL and that it TRACKS reliably along the vertical.
Click to expand...

Yes it does...if you are dialing the turrets. It does not if you are using mildots. For example, you could have the scope installed canted and the turret could move the reticle at a complementary angle, making it appear that both are working perfectly. However if you chose not to dial and instead use the subtensions on the reticle, the POI would be off.

Would you care to CLEARLY describe the "modified" version of the tall target method that WOULD work with mil-dot shooting?
Click to expand...

No I would not.

Click to expand...

Surprise, surprise, surprise. I really didn't think you would.
 
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I used a bubble level (actually two) mounted on opposite sides of the scope rail and a plumb bob string to level the scope. When I go out to shoot in my FT position sitting on a bum bag the reticle looks off about 5 degrees to the left when the rifle is bubble level. It must be because I tilt my head. My question is what effect, if any, would it have on the POI at 50 yards (with a 25 yard zero)) if I tilt the scope 5 degrees so it looks level to my eye?
Click to expand...

Another less common issue: It's possible that your vertical reticle is not in line with elevation turrets. Does it matter for your purpose?

Not all turrets track perfectly along the reticle. If tracking is critical, I verify by inserting a bore-sight laser at a distance where the laser dot is visible on the target. Maybe 10-25yds. Then I crank the elevation turret up and down to see if the laser dot tracks on the reticle. If it does, then I can align the reticle with bore and be comfortable that the turrets will track in line with the bore. If the laser dot does not stay on the reticle when twisting the elevation turret, and I only want to click for the next critical shots, then I would purposely cant the scope in the rings by the appropriate amount. By measuring the deviation angle of the laser from the reticle, I can determine the number of degrees to cant the scope in the rings.

If you want to be able to use both reticle stadia AND/OR clicking for the next shot, then you want a scope where the elevation turret tracks the reticle with high precision.

For those that just click, perfect tracking along the reticle is not required, though the tracking must be straight and intersect the bore-line.
 
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I know you have a lot of experience, but you never know where one might help. After having everything level and your eye relief set, the part where you are tightening down your scope can cause it to slightly come off level. I have had this happen often. I get the scope just tight enough to where I can move it back and forth, adjust eye relief then twist matching gun to scope, bubble to bubble. I will criss cross tighten the rings and check again. Sometimes at this point it will not match up perfectly anymore and I start over. Eventually it comes together for me though. If the bubble match, l then level to a target on the wall (target, and gun level also) and check the crosshair level to the target. Takes some time but always worth it to me.

Just saying how I do it and I usually come out with the result I want. I am pretty sure you might already do this and know that sometimes tightening the rings will take it off level slightly and may take a few times to get it, just trying to help. I do know that if the reticle is off just slightly I can tell immediately. 
 
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