Rifle Scopes The importance of correct leveled scope.....or not...?

[Spuhr]

After loads of questions and statements around the whole issue about scope levelling I made this little chart.
This is NOT a discussion if bubble levels are needed or expensive instruments when setting up your scope.
It's absolutely not a discussion if you have a weavermount from 1968 or the better mounts of today.

The idea is just to make a simple chart showing what counts and what just becomes uninteresting when it comes to levelling.

Sometimes people just spend in my opinion to much effort in checking so the scope is perfectly aligned over the bore when setting up the scope, but forget the testfire it for real.
A simple way of testing the correctness of the system is to fire 5 rounds at 100 meters, adjust 10 mils, shoot Another 5 rounds and first check so second Group is perfectly straight over the first Group and secondly at the correct distance betwen them in this case one meter.

opinions please?


Regards Håkan

 

[bogeybrown]

Very good info, thank you. As your intent is to avoid debate, I hope it's alright to simply comment that this data appears to confirm rather than debunk what I'd come to understand already about scope leveling. I saved the chart for future reference.

 

[Spuhr]

No, the debate is highly velcome!

But I just don't want any crap about it's me trying to sell mounts, because that is not the case.
I want an honest discussion about the matter.

And right now there is a lot of beliving in this matters, but few proofs or numbers about anything.

 

[Graham]

Provided that the scope is mounted properly, and almost level, then reticle level to the horizon appears to be the key. Of course there's little advantage to having the scope level to the rifle if you then cant the rifle. Of course, that's assuming you have a level horizon for reference. If you do a lot of angle shooting at longer ranges you will probably need a bubble-level because then you have no reliable horizon to use. In that case, level the scope to the rifle and use a mechanical aid.

I still don't use a bubble level, when prone on a bi-pod I level the rifle by looking at it.

 

[RonboF117]

I have read (but not confirmed by the company) that a certain scope manufacturer considers their scope in spec if it is within three degrees of true. This would correlate with your diagram showing a three degree tilt could be considered by many as an acceptable allowance.

For us, when we set up a new scope, we can easily establish the normal force within one stadia line. It's not for everyone but it's easy for us to do so we do it.

For advanced aiming systems that depend on an inertial reference platform, establishing the normal force requires DOV (deflection of vertical) added into the equation. This is the deflection of the normal force due to gravitational properties at the sensor location.

 

[tomcat mv]

I always mount my scopes using the plumb line method. I have a true level patio slab and use a Black & Decker help mate (or whatever it's called) check level, set rifle in vise or bi-pod, check level on the action then check vertical reticle alignment with plumb line which is about 40 yds. out. Screw it down and re-check best I can. Works for me as gravity doesn't lie.

 

[Chris@InterstateGuns]

Provided that the scope is mounted properly, and almost level, then reticle level to the horizon appears to be the key. Of course there's little advantage to having the scope level to the rifle if you then cant the rifle. Of course, that's assuming you have a level horizon for reference. If you do a lot of angle shooting at longer ranges you will probably need a bubble-level because then you have no reliable horizon to use. In that case, level the scope to the rifle and use a mechanical aid.

I still don't use a bubble level, when prone on a bi-pod I level the rifle by looking at it.

Frank had an interesting post basically the same thing but some more elements too it. I'm not a bubble level user either. My natural point of aim has me canting the rifle. To me NPA is more important. Wish I could explain this better.

 

[bruiz86]

In diagram B assuming erector tube is true to the reticle how would there then be any drift or offset? Straight up is straight up regardless how the rifle is canted if that cant stays consistent.

 

[wakajawaka]

I like to check using a vertical line drawn an the back of some old gift wrapping paper.
Put up the paper, draw the line using a carpenters level, stick an aiming dot at the bottom of the line.
Shoot, raise elevation and repeat.

 

[Spuhr]

In diagram B assuming erector tube is true to the reticle how would there then be any drift or offset? Straight up is straight up regardless how the rifle is canted if that cant stays consistent.

It's not very strange at all.
The bore line is not straight under the scope, and to explain it with an serious extreme so if:
you had you'r scope 1" to the left of the bore the hit would be spot on on 100 meters as you have it sighted in there.
But the hit would at 200 meters be 1" to left and at 300 meters be 2" to the left............

 

[hlee]

Now here is something that will really blow your mind. If the axis of your scope is offset by 3mm from the axis of your barrel, and your windage is +0 at 100 yards, at 1000 yards your windage will be off by 27mm. However, if your windage is +0 at 1000 yards, your windage will only be off by 2.7 mm (0.1") at 100 yards.

It pays to sight in at the longest distance possible...

 

[bruiz86]

that is false. The bore line is its own seperate axis as is the scope. The reticle moves within the scope body on the erector assembly to match the bores poi, not the entire scopes axis. The two remain in constant parallel as they are rigidly connected with the scope rings. The 1" offset never changes regardless the rotation of the rifle. Gravity is the other major constant. As long as the vertical erector tube and hopefully the reticle is true to gravity the rifle will track true.

It's not very strange at all.
The bore line is not straight under the scope, and to explain it with an serious extreme so if:
you had you'r scope 1" to the left of the bore the hit would be spot on on 100 meters as you have it sighted in there.
But the hit would at 200 meters be 1" to left and at 300 meters be 2" to the left............

 

[Spuhr]

Bruiz

It's not.
If you'r scope is 1" to the left of the bore, you will still sight it in so it hit center at 100 meters.
That means that the bullet is actually traveling from the right to the left, and that travel will not end just because it passes the 100 meters.
Therefor it will hit 1" to the left at 200 and 2" at 300 etc etc etc..........
And that is explaining figures B.

 

[bruiz86]

Incorrect again. Bullets only go straight they can't go right to left unless affected by wind. It is the reticle that moves to the bullet via the erector assembly.

Bruiz

It's not.
If you'r scope is 1" to the left of the bore, you will still sight it in so it hit center at 100 meters.
That means that the bullet is actually traveling from the right to the left, and that travel will not end just because it passes the 100 meters.
Therefor it will hit 1" to the left at 200 and 2" at 300 etc etc etc..........
And that is explaining figures B.

 

[Spuhr]

Maybe my English is insufficient for this discussion.
But here is a scetch.
This shows the situation from above, over the shooter, when a scope is mounted besides the bore.
The bullet is travling straight in it's direction, and you are looking in Another direction.
The paths of the bullet and the line of sight are not parallel unless you sight your gun in to hit at 1" to the right at 100 meters.........

 

[hlee]

But, line of sight is a, wait for it, a line. In order for line of sight to intersect the arc of the bullets trajectory, it is angled up/down/left/right relative to the fixed axis of the bore and the scope. Relative to the axis of the bore, and ignoring any effects other than gravity, the bullet travels "straight" and only moves down in a plane defined by gravity and the bore axis. If line of sight is also contained within this plane (errector assembly mounted directly over the bore axis), then bruis is correct. However, this discussion is about when the scope is mounted such that the axis of the line of sight is not in this plane. In this case, the line of sight can only intersect the plane (and hopefully the arc of the bullets trajectory) at a single point. At all other points along the line of sight the point of aim will be left or right of point of Impact. Because line of sight is straight (it is a line) and because we can simplify the scope/rifle junction to 2 points connected by a line, the math is linear and simple to calculate. If the scope is offset to the left of the bore by 1" and windage is 0 at 100 yards, then the bullet will impact the target 1" left of poi at 200 yards. If you do not believe this then take your rifle to the range and shoot it with the scope laid over 90 EEG. Sight it in at 100 yards then move to 200 yards and see what your poi shift is. I'll bet dollars to donuts it is off by the scope height.

 

[bruiz86]

The scopes line of sight never changes. Your actually moving the reticle within the scopes field of view to match that of the bullet.

 

[Spuhr]

Bruiz.

If you please look again at figure B you see that the scope is mounted to the right of the bore!
That is what will happend if you mount your scope canted but hold it vertical when shooting.
Then will the bore not be vertially under the scope and the result till be an increasing drift from the target.
So please look over the figures again, because you are wrong.
But as said Before, the issue might be my bad English when it comes to the explanation.

 

[hlee]

Yes, but the center of the reticle and the target describe a line. If this line is not within the plane defined by gravity and the bore axis (the plane that contains the bullet's arc of trajectory), then it can only intersect the bullets arc of trajectory at a single point. A high school geometry student could draw this better than I can type it.

 

[bruiz86]

your drawing is misconstrued. The scope is not canted to the bore unless a canted base is used. they both are parallel or as close as the machined action, base, and rings can allow. The reticle through windage and elevation adjustments moves to match the rifles poi.

Maybe my English is insufficient for this discussion.
But here is a scetch.
This shows the situation from above, over the shooter, when a scope is mounted besides the bore.
The bullet is travling straight in it's direction, and you are looking in Another direction.
The paths of the bullet and the line of sight are not parallel unless you sight your gun in to hit at 1" to the right at 100 meters.........

 

[hlee]

Spuhr, your English is fine and your diagrams are descriptive and concise. Something -else- is missing...

 

[Spuhr]

the scope can be canted two ways!
It can be canted to compensate for the trajectory such as a 20 moa mount does.
But it can also be canted in the rotation.
(maybe here is a big grammatical error)
But a scope that is rotated 3 degrees when mounted in it's rings and when you shoting with it, held correct vertical, that scope will leave the barrel not vertical under the scope but offset to the side, just like figure B shows.

 

[hlee]

Bruiz86, please try the following exercise...

Sight your gun in at 100 yards, with the scope 90deg off of vertical. To the left will likely be the most comfortable if you are a right hander, to the right for a lefty. Then, shoot at 200, 300, and 400 yards. Please report back your findings.

It should be notes that 27 mm is roughly 1 inch, so a 3deg cant (which is obvious to the naked eye) is insignificant at 1000 yards given other factors (like WIND).

 

[redirt78]

I use holdovers out to 600 all the time with my 308 and G2 reticle. If my reticle isn't level then my shots will be way off. A lot of places that I shoot can be deceiving as far as holding the reticle level. Sometimes I test myself and try to level the crosshairs without using the bubble then check myself with the level and sometimes I am off a good bit. I'll probably get better with practice and not need the level as much but I do use it a lot. I usually dial elevation when shooting past 600 and don't feel it is as important when using the center of the reticle.

 

[bruiz86]

Yes you are correct. If you have your 3mm offset in the diagram but then hold the rifle vertical thus creating a 3mm canted reticle you will have drift because the elevation erector is not true to gravity. if you take that same setup but hold the rifle with the 3mm of cant thus making the reticle and erector level with gravity you will be fine at any range. The scope and barrel are one unit tied together. If you move the "3mm canted rifle with vertical reticle" any direction the scope moves an equal amount. A bullet is only affected my force which would be mainly gravity and wind. If your bullet it true to gravity and your reticle is true to gravity how can the deviate from the same vertical line?

Bruiz.

If you please look again at figure B you see that the scope is mounted to the right of the bore!
That is what will happend if you mount your scope canted but hold it vertical when shooting.
Then will the bore not be vertially under the scope and the result till be an increasing drift from the target.
So please look over the figures again, because you are wrong.
But as said Before, the issue might be my bad English when it comes to the explanation.

 

[Spuhr]

Bruiz

Nope now you are twisting it again.....

The scope is held true vertical, but as the scope not have been mounted correctly the bore is not under the scope, but offset.
that is what is shown in figure B.

 

[bruiz86]

Cant is angular not linear. A 5 degree cant at 100 yards is the same as a 5 degree can't at 1000. A better comparison is MOA. 5 moa is .08 degrees at 100 yards and 1000. The apparent drop only happens linearly due to gravity as you move away from that angle.

 

[bruiz86]

I did this exact exercise several years ago just convince myself. Took my 308 and purposely set the rifle on a bipod canted it roughly 20 degrees right as I'm a lefty. I rotated my butt pad so it was a little more comfortable to shoot even though cheek weld sucked. I used a plumb bob to align the reticle vetical and a scope mounted bubble level and torqued everything. took quite a bit of adjustment to get the 100 yard zero. From there dialed in 10 moa to get to 500 yards and fired a group. It held vertical perfectly. I was honestly planning on missing the target completely. Since then its been pretty easy to get a comfortable natural poa and plumb the reticle from there. Gravity doesn't lie.

Bruiz86, please try the following exercise...

Sight your gun in at 100 yards, with the scope 90deg off of vertical. To the left will likely be the most comfortable if you are a right hander, to the right for a lefty. Then, shoot at 200, 300, and 400 yards. Please report back your findings.

It should be notes that 27 mm is roughly 1 inch, so a 3deg cant (which is obvious to the naked eye) is insignificant at 1000 yards given other factors (like WIND).

 

[Pinecone]

If you have the reticle vertical and dial up on a 100 yard target, it WILL go up a vertical line above the 100 yard POI. Assuming a quality scope.

BUT, if you actually move out to longer ranges, then you will see the POI move in the direction from the bore to the scope, increasing at greater ranges.

UNLESS, you have a 3mm off axis scope mount, so you zero your rifle at 100 yards with the POI 3mm off to the side from scope centerline to bore centerline. Then, the POI will remain 3mm to the side at all ranges.

I don't know about you super shooters, but I can't hold to 3mm at a 1000 yard target.

Bottom line, the most important thing is that the reticle is vertical/horizontal to the WORLD when you take the shot. And for that reason, I run a SCOPE MOUNTED level. That is leveled to the SCOPE, not the rifle.

 

[para1505]

Damn you all sound like sheldon and Amy on the Big Bang theory

 

[997/2man]

This is a great thread. I love when we do gun brain-ups!


Theoretical v. Practical accuracy

0-300 yards using a 14.5"-16" .223 rifle with non match ammo, I'm happy with 2.5" groups shot rapid fire.

0-600 yards using a 16" .308 rifle with non match ammo, I'm happy with 2.5" groups shot rapid fire.

Using a 1-6X or a 1-8X I honestly can't hold any closer, not enough magnification, reticles are usually too coarse.

The long range bench guys use high mag scopes (10X+) at long range(600yards+) may be worried about these numbers, for me, not really a factor.

Level the scope using feelers, Mr. Spuhr's wedge, bubble levels, etc. Things like good hold, good trigger control, consistency, correct stance or position will go farther in yielding better groups from a practical standpoint than worrying about 0.1 degree of vertical scope misalignment.

 

[Pinecone]

The point is, it is not important to level the scope to the rifle, it is important to level the scope to the world when you are shooting.

 

[Aeon1]

Frank had an interesting post basically the same thing but some more elements too it. I'm not a bubble level user either. My natural point of aim has me canting the rifle. To me NPA is more important. Wish I could explain this better.

I'm fully with you. I never understand why people must level everything and fight NPA. For me good NPA creates more consistency.

The discussion I think you are referencing is here: http://www.snipershide.com/shooting...rth-level-scope-gun-then-level-set-earth.html

 

[bogeybrown]

The point is, it is not important to level the scope to the rifle, it is important to level the scope to the world when you are shooting.

THIS!
The science of all this can clearly cause a lot of confusion. The no-shit reality is if your reticle is levelled to the world (I use a plum line) and you then cant your rifle (and by extension your scope) there will be a POI shift at distance.
Depending on which rifle/scope I'm using, I will either shoot holds or will dial elevation and hold windage. Even when dialing elevation so there is no exaggerated effect at the bottom of the reticle I have seen countless times where I am on target and will do a final check of my scope level and see I have a TINY bit of cant, as I correct the cant the POA in my scope changes dramatically. If I hadn't corrected for the cant I would have missed, and at 800m plus I would have instantly blamed it on a poor wind call when in fact it was scope/rifle cant.
I'm hoping Euclid and Pythagoras and whoever else agree with this in theory, because the bullet doesn't lie.

 

[tylerw02]

What happens at 1500 meters or 2000?

 

[hlee]

What happens at 1500 or 2000 yards is a mathematical extrapolation of the relationship between the sight in distance and the offset of the rifle/scope. In the example above, with a 3 mm offset and a sight in distance of 100 yards- and assuming that the shooter is good enough to tell the difference between dead center and of by 0.1 inch- the poi will be off by ~1.5" at 1500 and ~2" at 2000 yards (left or right) relative to the poa. But, these measurements are so small that they are likely to never be noticed by the vast majority of shooters. The mathematical reality is an insignificant factor within the context of aligning a scope to a rifle bore- assuming that the difference is small enough to go un noticed by the naked eye AND assuming that the reticle is always held perpendicular to the pull of gravity. But, if you sight in with a perpendicular reticle and shoot at distance with a canted reticle (or visa versa), your poi shift can be much greater than in the example above...

 

[Pinecone]

I'm fully with you. I never understand why people must level everything and fight NPA. For me good NPA creates more consistency.

The discussion I think you are referencing is here: http://www.snipershide.com/shooting...rth-level-scope-gun-then-level-set-earth.html

The point that was made at the beginning was that do not worry about canting the rifle for NPA, as long as the scope (reticle) is level when you shoot. So if your NPA is canted, set your scope up to not canted at your NPA.

This is why I prefer a scope mounted level, as that is what is important. I can rotate the scope 90 degrees to the rifle, level the scope (reticle using the scope mounted level) and shoot for hits.

 

[Pinecone]

THIS!
The science of all this can clearly cause a lot of confusion. The no-shit reality is if your reticle is levelled to the world (I use a plum line) and you then cant your rifle (and by extension your scope) there will be a POI shift at distance.

Exactly. BUT, if you naturally cant your rifle, and mount the scope so IT is level when you hold the rfile, it will work fine.

Canting the rifle does not affect the POI by much (VERY slightly). Canting the scope is a BIG DEAL.

 

[hlee]

The point that was made at the beginning was that do not worry about canting the rifle for NPA, as long as the scope (reticle) is level when you shoot. So if your NPA is canted, set your scope up to not canted at your NPA.

This is why I prefer a scope mounted level, as that is what is important. I can rotate the scope 90 degrees to the rifle, level the scope (reticle using the scope mounted level) and shoot for hits.

What is your scope height?

 

[bogeybrown]

Exactly. BUT, if you naturally cant your rifle, and mount the scope so IT is level when you hold the rfile, it will work fine.

Canting the rifle does not affect the POI by much (VERY slightly). Canting the scope is a BIG DEAL.

RIGHT. When mounting a scope I do my best to ensure my rifle is level to the world, but I don't obsess over it because a canted rifle really isn't that crucial. I completely obsess over my scope being level to the world because once it's mounted and locked down whatever I do to the rifle will effect the orientation of the scope. If my natural position required a slight cant to the rifle, then I'd mount the optic level to the world on the canted rifle.
By extension, I would imagine that if you don't level your scope to the world, but are able to shoot in the same exact position as you zeroed then you'd be fine. On a square level range with square level targets I guess this may be do-able. On a field course shooting from uneven ground at crooked targets on un-even ground where there is no true reference for level or plumb I don't know that I could guarantee the same exact orientation of my weapon every time, hence the scope level. The level is the truth, and it eliminates the variable of scope cant, because I've proven to myself that my eyes can deceive me.

 

[Dynamic Response]

Anyone figure out how much scope cant you need to counter the spin drift.

 

[bogeybrown]

Anyone figure out how much scope cant you need to counter the spin drift.

Hahahahaha. Excellent idea. With some of the brain power in this discussion so far someone may be able to. My math sucks so I'm limited to what I see happening and I just have to trust that somewhere there's a scientific explanation.

 

[garandman]

I like to check using a vertical line drawn an the back of some old gift wrapping paper.
Put up the paper, draw the line using a carpenters level, stick an aiming dot at the bottom of the line.
Shoot, raise elevation and repeat.

Saw this method on a recent TV gun show.

Can anyone confirm its usefulness?

I don't really care about the theoretical discussion, just looking for a practical way to avoid scope cant.

 

[Lowlight]

In the Northern Hemisphere you can just use a Left Hand Twist Barrel and negate most of SD, especially at ELR Distances.

The amount of SD is not so cut and dry as people throw a bit of their own "drift" in there depending how they break the trigger.

Ask a left handed shooter how much and it's libel to go the other way.

I believe David Tubb was canting his reticles to remove SD but he is one of the few guys who can actually see the difference, as opposed to adding flat rate values with the one size fits all approach 90% of the people out there use, calling it SD.

 

[pmclaine]

Anyone figure out how much scope cant you need to counter the spin drift.

Just attach Springfield M1903 irons on your rifle. The rear sight leaf slider will adjust automatically for spin drift.

Note how the leaf is milled so that as the slider is moved up in elevation the peep and sight notches move to the left. Engineers had a lot of time on their hand prior to the internets arrival. The sight on the left has at least 5 sighting choices. The one in the middle is a USMC #10 and the one to the right is a WWII replacement with standard peep and they dispensed with the volley notch at the top of the leaf. I guess they realized 2900 yards with a 30-06 was not worth it to the war effort.

I wonder if the Marines were screwed when they went south of the equator with their 03's on Guadalcanal and found their spindrift calculations were a mess. We might have lost the war.

 

[Lowlight]

Saw this method on a recent TV gun show.

Can anyone confirm its usefulness?

I don't really care about the theoretical discussion, just looking for a practical way to avoid scope cant.

You mean like this

On the left, those dots are 1 mil apart, you can level it with a level and then move the scope up to test the adjustments, 7 mils worth

From a video of mine

 

[garandman]

You mean like this


On the left, those dots are 1 mil apart, you can level it with a level and then move the scope up to test the adjustments, 7 mils worth

From a video of mine

View attachment 13948

The idea ( on the TV show) was more to see if the rounds impacted off the line.

 

[Lowlight]

Ah, that is what the line is for, you can adjust the scope holding the bottom dot, and dial it up, and then see if it moves, or hits the dot.

There is more than one purpose, it just depends how you use it... there is still a "line" there.

 

[Burton]

This is a very interesting post, and I have enjoyed the debate.

I saved the chart for further reference, thanks for sharing.

If anyone has ever sighted in an winchester lever action with a side mount scope, or an SKS or anything with a scope that is mounted over the side of the rifle, then you probably understand what Spuhr is talking about. If you could extend your vertical stadia line down to the bore and it does not intersect the center line of the bore, then you are asking for trouble unless you only shoot at ranges where the deviation does not matter.

Keep posting stuff like this, keeps the mind of a shooter sharp, and causes one to think about the physics and trigonometry behind precision shooting.

Thanks for sharing.

 

[keith jones]

Lowlight where can you pick some of those targets up at?