How to mount long-range optic?

[Pwilliams1]

Subscribing after pissing myself off with the last one I tried

 

[axshon]

In terms of leveling your scope to the rifle, a quality scope, rings, base and rifle will always result in your scope's crosshairs being aligned with your bore centerline. This is aligned but not level which is a good starting point but not the end of the story.

Consider this data from Bryan Litz "Accuracy and Precision for Long Range Shooting":

Paraphrasing the book; the cant of your scope will have very little effect on POA v/s POI at short distances. At longer distances however it can have a significant effect. This is because you are calculating the drop of your projectile over distance and then adjusting your scope's elevation turret along a line that is not the same as the trajectory. This is the same as canting your entire rig if it everything is level.

According to Litz:
- .308 SMK 175 at 2600 fps MV
- 3 degrees of right cant error
- 200 yard error: 0.6 inches
- 500 yard error: 4.4 inches
- 1000 yard error: 24.7 inches

Not considering wind, on a 48" target, center mass point of aim, at 1000 yards that amounts to a 100% miss rate.

The farther out you shoot, the more important it is to have the scope leveled perfectly to the rifle and the world. If you have no ability to measure level with the world around you (aka hunting) and you can make an ethical shot at long range, consider a scope level. I use one because when I'm practicing I will level the rifle on the bipod and then check the level to see how close I got by eyeing it. I can always get inside 3 degrees but I am sometimes off more than I expected.

 

[Pinecone]

so what do you suggest as far as some steps on setting this all up?

what is the procedure, how do you do it?

how do you ensure the scope is level to the world when you shoot?

Mount the scope with the rings slightly loose. Hang a plumb bob and some convenient distance, but a reasonable distance away. Shoulder rifle with eyes closed, open and look at plumb bob. If vertical reticle is even with the plumb bob, you are GTG. Otherwise, rotate the scope and repeat. You are looking for your natural hold and the reticle being vertical. It for a primarily long range rifle, you can do this prone.

You install a scope level that you can adjust so that when the scope reticle is level, the level shows level. Then, as you adjust for your shot, you check the level and ensure the reticle (level) is level with the world.

 

[Pinecone]

In terms of leveling your scope to the rifle, a quality scope, rings, base and rifle will always result in your scope's crosshairs being aligned with your bore centerline. This is aligned but not level which is a good starting point but not the end of the story.

Consider this data from Bryan Litz "Accuracy and Precision for Long Range Shooting":

Paraphrasing the book; the cant of your scope will have very little effect on POA v/s POI at short distances. At longer distances however it can have a significant effect. This is because you are calculating the drop of your projectile over distance and then adjusting your scope's elevation turret along a line that is not the same as the trajectory. This is the same as canting your entire rig if it everything is level.

According to Litz:
- .308 SMK 175 at 2600 fps MV
- 3 degrees of right cant error
- 200 yard error: 0.6 inches
- 500 yard error: 4.4 inches
- 1000 yard error: 24.7 inches

Not considering wind, on a 48" target, center mass point of aim, at 1000 yards that amounts to a 100% miss rate.

Yeap.

The farther out you shoot, the more important it is to have the rifle leveled perfectly to the rifle and the world. If you have no ability to measure level with the world around you (aka hunting) and you can make an ethical shot at long range, consider a scope level. I use one because when I'm practicing I will level the rifle on the bipod and then check the level to see how close I got by eyeing it. I can always get inside 3 degrees but I am sometimes off more than I expected.

If you mean scope leveled to the rifle, nope. Scope needs to be leveled to the WORLD. You could hold the rifle at a 45 degree cant with the scope level to the world and still make the hits.

 

[amoebicmagician]

as has been said, just find a level setup for you gun, set a plumb bob about 30 yards away, and align reticle to that. Has never let me down, with proof in the pudding out to thousands of yards.

Be aware however, that if you do not hold your rifle exactly parallel to the ground with your crosshairs perpendicular to same when shooting, your shot can be considerably off.

For a long time I couldn't figure out why I was getting shit groups until I realized that if I am canting the rifle to the right, my vertical windage is going to throw my shot far to the right due to the parabolic trjactory being discarded due to gravity no longer being a factor on that axis.

 

[amoebicmagician]

In terms of leveling your scope to the rifle, a quality scope, rings, base and rifle will always result in your scope's crosshairs being aligned with your bore centerline. This is aligned but not level which is a good starting point but not the end of the story.

Consider this data from Bryan Litz "Accuracy and Precision for Long Range Shooting":

Paraphrasing the book; the cant of your scope will have very little effect on POA v/s POI at short distances. At longer distances however it can have a significant effect. This is because you are calculating the drop of your projectile over distance and then adjusting your scope's elevation turret along a line that is not the same as the trajectory. This is the same as canting your entire rig if it everything is level.

According to Litz:
- .308 SMK 175 at 2600 fps MV
- 3 degrees of right cant error
- 200 yard error: 0.6 inches
- 500 yard error: 4.4 inches
- 1000 yard error: 24.7 inches

Not considering wind, on a 48" target, center mass point of aim, at 1000 yards that amounts to a 100% miss rate.

Yeap.



If you mean scope leveled to the rifle, nope. Scope needs to be leveled to the WORLD. You could hold the rifle at a 45 degree cant with the scope level to the world and still make the hits.

this is exaclty what I was trying to explain. Your elevation adjustment means your cross hairs are not directly in line with your bore but on a slightly downward slant to make up for gravity and allow your point of aim to follow the resulting parabolic arc generated by firing your bullet at a slihgtly upward angle. Get it?

 

[axshon]

If you mean scope leveled to the rifle, nope. Scope needs to be leveled to the WORLD. You could hold the rifle at a 45 degree cant with the scope level to the world and still make the hits.

Yes, that's what I meant, fixed in post above.

Not sure I understand what a 45 degree cant on a scope leveled to the world would look like. If you mean holding your rifle at 45 degrees and then leveling your scope to a plumb line then for every increment of elevation you would be inducing an equal amount of horizontal distance from the trajectory. The bullet leaves the rifle heading up (then down) as it relates to the centerline drawn from the center of the bore through the scope reticle centerline regardless of which direction the reticle lines go. If you draw that line straight up and down that should be the arc of your bullet. If you adjust elevation and your scope is not leveled to the rifle, you are not moving the reticle along that centerline. All adjustments will be incorrect.

 

[lsw]

Seems like someone could have designed a self-leveling reticle by now...

 

[Pinecone]

Yes, that's what I meant, fixed in post above.

Not sure I understand what a 45 degree cant on a scope leveled to the world would look like. If you mean holding your rifle at 45 degrees and then leveling your scope to a plumb line then for every increment of elevation you would be inducing an equal amount of horizontal distance from the trajectory. The bullet leaves the rifle heading up (then down) as it relates to the centerline drawn from the center of the bore through the scope reticle centerline regardless of which direction the reticle lines go. If you draw that line straight up and down that should be the arc of your bullet. If you adjust elevation and your scope is not leveled to the rifle, you are not moving the reticle along that centerline. All adjustments will be incorrect.

No, if the scope is leveled to the WORLD, the scope elevation tracks up and down like the bullet.

If you level the scope to the rifle and then hold a cant, the bullet still travels up and down to the world, but if you change the elevation on the scope, the "elevation" will include some windage.

If you did mount a scope at 45 degrees to the rifle, and held the rifle at a 45 degree cant, the elevation tracking will still be up and down with respect to the world and the bullet.

The point is, the bullet reacts to gravity. NOT the rifle orientation.

Now, if the scope is significantly off to the side of the bore, you need to be careful with seeting your POA/POI for windage.

If the scope axis is say 2 inches (A LONG way off) to the left of the bore, if you precisely zero the rifle at 100 yards, then at 200 yards, the bullet will impact 2 inches to the RIGHT of the point of aim. And at 300 yards, it will be 4 inches off.

BUT, if you sight in your rifle so that the POI is 2 inches to the left of POA at 100 yards (same as the scope offset) the POI will remain 2 inches left at 200, 300, 500, 1000, etc.

 

[Pinecone]

The bullet leaves the rifle heading up (then down) as it relates to the centerline drawn from the center of the bore through the scope reticle centerline regardless of which direction the reticle lines go. If you draw that line straight up and down that should be the arc of your bullet. If you adjust elevation and your scope is not leveled to the rifle, you are not moving the reticle along that centerline. All adjustments will be incorrect.

NO, the bullet goes "up" based on the bore to reticle, but it goes down based on GRAVITY.

 

[INMY01TA]

ONE MORE TIME.

It does NOT matter if the scope is level to the rifle.

The ONLY thing that matters is that the scope is level to the WORLD when you shoot.

Many people hold the rifle with a slight cant due to their shoulder pocket. If you level to the rifle, and cant it, as you change elevation, the shots will walk off to the side.

And the scope level should be leveled to the RETICLE, not the rifle.

What if you're using a 20moa base?

 

[Pinecone]

Same thing

Of course, if you cant the rifle a LOT, the 20 MOA base will have other issues. But most people cant the rifle a few degrees.

Basically, with any cant, part of that 20 MOA will show up as windage error, so you will have to use some of your windage to dial that out. And it would also slightly reduce your elevation gain with the base, as a small part of the 20 MOA would be lost.

If you canted 45 degrees, you would only effectively have about a 14 MOA elevation. But would also have 14 MOA of windage offset.

 

[Magnumite]

No, if the scope is leveled to the WORLD, the scope elevation tracks up and down like the bullet.

If you level the scope to the rifle and then hold a cant, the bullet still travels up and down to the world, but if you change the elevation on the scope, the "elevation" will include some windage.

If you did mount a scope at 45 degrees to the rifle, and held the rifle at a 45 degree cant, the elevation tracking will still be up and down with respect to the world and the bullet.

The point is, the bullet reacts to gravity. NOT the rifle orientation.

Now, if the scope is significantly off to the side of the bore, you need to be careful with seeting your POA/POI for windage.

If the scope axis is say 2 inches (A LONG way off) to the left of the bore, if you precisely zero the rifle at 100 yards, then at 200 yards, the bullet will impact 2 inches to the RIGHT of the point of aim. And at 300 yards, it will be 4 inches off.

BUT, if you sight in your rifle so that the POI is 2 inches to the left of POA at 100 yards (same as the scope offset) the POI will remain 2 inches left at 200, 300, 500, 1000, etc.

A canted rifle shoots in the direction it is canted. A scope is deigned to work on a plane perpendicular to and in alignment with gravity (windage and elevation respectively). Aligning the scope relative to the vertical plane on a vertically held firearm will cause the bullet to coincide with windage at all times since gravity is always pushing/pulling downward in the vertical plane. A scope mounted vertical on a canted firearm will cause the line of sight and barrel axis to intersect on two planes, not one as with a properly mounted scope. The longer the range means greater deviation from both intended planes.

 

[Pinecone]

Not exactly. As I stated before, with a canted rifle, you can shoot parallel to the line of sight for windage and not have that issue with two planes.

But if your natural hold is canted, and you mount your scope level with the rifle, not the world, as you dial your elevation, you will be adding windage and miss.

Take our 45 degree cant. If the scope is leveled with rifle, if you dial in 10 MOA of elevation, you will get 7 MOA of elevation and 7 MOA of windage. If that 10 MOA is from 100 yards to 500 yards, you would miss to the side by 35 inches.

 

[Magnumite]

The only way there would be consequence is if the line of sight and the bore coincide on one plane. What you are proposing is the same as the offset scope mounts for some firearms. Or a front sight of a pistol which is biased to one side of the bore. In both od those setups, at progressive distances, you can see the group converging toward, intersecting, then diverging from the line of sight. I know of no shooter setting up precision guns without the bore and sights/optics coinciding on the vertical plane.

Additionally, holdover/under, elevation, and windage adjustments are more consistent with a properly aligned optics/sights. One doesn't need to compensate for adjustments to bore/line of sight deviation.

 

[new_shooter]

Good thread. I've not considered how cant could affect long range placement. (I've not had a chance to shoot longer ranges.) Now that the thought is in my head I can see how that would have a significant effect. Thanks all.

 

[Pinecone]

The only way there would be consequence is if the line of sight and the bore coincide on one plane. What you are proposing is the same as the offset scope mounts for some firearms. Or a front sight of a pistol which is biased to one side of the bore. In both od those setups, at progressive distances, you can see the group converging toward, intersecting, then diverging from the line of sight. I know of no shooter setting up precision guns without the bore and sights/optics coinciding on the vertical plane.

Additionally, holdover/under, elevation, and windage adjustments are more consistent with a properly aligned optics/sights. One doesn't need to compensate for adjustments to bore/line of sight deviation.

Hmm, I guess you have never looked at Ed Shell's posts on the subject.

Or David Tubbs.

Or any number of long range shooters.

Reticle MUST be vertical with the WORLD.

If you have significant offset from the bore, you can simply sight in your rifle to impact that far to the side of the POA, and it will remain that distance off.

So if your scope axis is 2 inches to the left of the bore, if you sight in to impact at 2 inches left at 100 yards, and still 2 inches left at 1000 yards, and at 1500 yards.

NO convergence and divergence of the POI versus POA.

 

[Magnumite]

So you're saying the scopes can be adjusted parallel to the bore. Then the parallel distance between them is constant. So a 30 degree left cant with a two inch difference in center lines, while extreme, would be a 1" distance from the trajectory. It is then added or subtracted from windage dependent on direction of hold. Correct?

Hmm... Like the spotter scope and telescope for astronomy. There are/were some artillery sights like that, I believe.

 

[Pinecone]

Yes, basically.

If you were to sight in at 100 yards to make POI=POA, at 200 yards, the POI would be 1 inch RIGHT of POA. At 300 yards it would be 2 inches right, and at 1000 yards, it would 9 inches right.

BUT, sight in so that POI is 1 inch left (same as offset) at 100 yards and it will stay 1 inch left at all ranges.

 

[Magnumite]

got it.