Scope Leveling; Instruction that ran counter to everything I thought I knew

[IBA_270]

Super interesting exchanges here. FWIW...the Applied Ballistics app can account for cant (offset) if I'm not mistaken...

I had no idea my 6th post would start this. Sorry about that...

 

[omicron1792]

I will try to explain cant quickly. Using extremes.

The bullet leaves the barrel 1.5”-2.75” under your scope. Most rifles are set up between 1.75-2.25”. That is significant. We are not shooting laser beams. We are shooting a bullet that leaves the rifle and immediately pulled by gravity in an arch until it stops.

If you took your rifle and laid it on its side with a 90 degree cant and dialed 8.4mil on your scope for a 1000yd target. Would you expect to hit your target? Absolutely not. Even if you had perfect fundamentals and great npa you will miss your target by a lot.

Well now break the same shot with a 45deg cant…you will still miss by a ton. Now try 15deg cant. Still missing. As you get closer to 0deg can’t you will start seeing less drift on your POI.

This is just basic physics.

That’s not exactly right.

Many mount a red dot or second optic on 45 or 90 offset from gun being up right. Doesn’t matter at all.

If you laid your gun on its side, and mounted th me scope at 90 degrees, then the bullet would still behave with regards to gravity, not the orientation of the gun to the ground. Think of a gangster turning pistol to the side as he shoots.

I’m not saying it’s the best way to do it. But as long as the reticle is plumb with regards to up and down then the bullet will behave the same when it leaves the barrel.

 

[omicron1792]

What about windage?

Wind would affect the bullet the same regardless of the roll of the rifle. And your windage changes on scope would do the exact same thing as long as your scope is level to the ground.

See this picture. Special forces offset optics all the time. The bullet is still shooting straight and dropping towards the earth curve.

 

[IBA_270]

That’s not exactly right.

Many mount a red dot or second optic on 45 or 90 offset from gun being up right. Doesn’t matter at all.

If you laid your gun on its side, and mounted th me scope at 90 degrees, then the bullet would still behave with regards to gravity, not the orientation of the gun to the ground. Think of a gangster turning pistol to the side as he shoots.

I’m not saying it’s the best way to do it. But as long as the reticle is plumb with regards to up and down then the bullet will behave the same when it leaves the barrel.

That was a point I was going to make...er...ask; if you take a barreled action and clamped to a table and fired it, then rotated it 90 degrees, then rotated...the bullet will perform exactly the same. It won't land in the same spot of course, that's why we zero the optic. based on the orientation of the action bedded in the stock and fired from our shoulder.

 

[Molon Labe]

That’s not exactly right.

Many mount a red dot or second optic on 45 or 90 offset from gun being up right. Doesn’t matter at all.

If you laid your gun on its side, and mounted th me scope at 90 degrees, then the bullet would still behave with regards to gravity, not the orientation of the gun to the ground. Think of a gangster turning pistol to the side as he shoots.

I’m not saying it’s the best way to do it. But as long as the reticle is plumb with regards to up and down then the bullet will behave the same when it leaves the barrel.

If you were shooting at a target that was the same range as your zero that is correct.

I should have included “assuming a 100yd zero” my bad.

If you vice that barreled action and rotate rifle your point of aim will be different if you are shooting at any distance other than your zero. The bullet will have the same point of impact but it will not be at your point of aim.

 

[Formidilosus]

A lot of yall are either conflating multiple things, or confused about what is being discussed.

Wind would affect the bullet the same regardless of the roll of the rifle. And your windage changes on scope would do the exact same thing as long as your scope is level to the ground.

See this picture. Special forces offset optics all the time. The bullet is still shooting straight and dropping towards the earth curve.

View attachment 782554

Offset dots are rotated so that the bore is directly under the dot.

That is not what is happening when you have a reticle level to the earth, but canted to the rifle.


In any case, unless extremely canted to the rifle, the effect on windage and elevation is minimal and hard to detect. However, it still is a sub par method for the reasons that I outlined.

 

[PredatorSlayer]

Wind would affect the bullet the same regardless of the roll of the rifle. And your windage changes on scope would do the exact same thing as long as your scope is level to the ground.

See this picture. Special forces offset optics all the time. The bullet is still shooting straight and dropping towards the earth curve.

View attachment 782554

I am talking about windage adjustments for long range to compensate for the cant of the reticle. Short range the difference is immaterial which is why it doesnt matter for combat. Most of the time the offset optic is a red dot with a short range zero. I even have an offset red dot on my bolt coyote gun for close shots.

 

[sndmn11]

Some body mock up a model with an arrow shaft and a rod tip then close this thread.

 

[Ice-kub]

Some body mock up a model with an arrow shaft and a rod tip then close this thread.

I think this would be the easiest way to show what's actually going on

 

[omicron1792]

A lot of yall are either conflating multiple things, or confused about what is being discussed.






Offset dots are rotated so that the bore is directly under the dot.

That is not what is happening when you have a reticle level to the earth, but canted to the rifle.


In any case, unless extremely canted to the rifle, the effect on windage and elevation is minimal and hard to detect. However, it still is a sub par method for the reasons that I outlined.

Not disagreeing with any of that.

There were people early on that were insinuating the bullet would come out sideways. Thats not the case.

 

[Marbles]

I will try to explain cant quickly. Using extremes.

The bullet leaves the barrel 1.5”-2.75” under your scope. Most rifles are set up between 1.75-2.25”. That is significant. We are not shooting laser beams. We are shooting a bullet that leaves the rifle and immediately pulled by gravity in an arch until it stops.

If you took your rifle and laid it on its side with a 90 degree cant and dialed 8.4mil on your scope for a 1000yd target. Would you expect to hit your target? Absolutely not. Even if you had perfect fundamentals and great npa you will miss your target by a lot.

Well now break the same shot with a 45deg cant…you will still miss by a ton. Now try 15deg cant. Still missing. As you get closer to 0deg can’t you will start seeing less drift on your POI.

This is just basic physics.

15 degree cant with 2.25 inch scope height

Sin(15)x2.25in=0.58in offset from bore.

100 yard zero, on the horizontal axis you have a 0.58 shooters MOA angel. At 200 yards your shot will be off by 0.58 shooters MOA, it will still be off by 0.58 shooters MOA at 500 yards, but that is now a 2.33 inch error, at 1000 yards that is a 5.24 inch error. You could dial it out by offsetting your zero at 100 yards by 1/2 MOA.

Zero at 200 yards and the error becomes 0.29 shooters MOA. Zero at 300 and it becomes 0.19 shooters MOA.

In hunting, the offset at 15 degrees of cant is meaningless. However, why the hell anyone would want to shoot a rifle canted at 15 degrees is beyond me.

A 5 degree cant with 2.25 inch scope hight gives a 0.2 inch offset, so less than 1/4 MOA with a 100 yard zero, meaning it is meaningless.

45 degree cant with 2.25 scope hight is 1.59 inches of offset, with a 100 yard zero, you will be missing by 1.59 inches at 200 yards, more than is acceptable for sure, but not what I would call a ton.

The math is pretty simple, just working with trig.

Now, if the reticle is canted things get more complicated.

Trig problem number 1

Trig problem number 2

 

[Tnc]

Because the scope and the barrel are no longer on the same plane and a bullet still rises when it is fired from a barrel regardless of how much it is canted.

It’s the year of our Lord two thousand and twenty four. Please tell me you don’t think a bullet rises when it is fired from a barrel.

 

[Marbles]

It’s the year of our Lord two thousand and twenty four. Please tell me you don’t think a bullet rises when it is fired from a barrel.

Well, if the barrel is canted upwards......



But I had the same thought when I read that.

 

[Tnc]

Well, if the barrel is canted upwards......



But I had the same thought when I read that.

Even canted upwards, gravity takes over the second it leaves the barrel and it starts dropping from the line of the bore.
(I know you know this but I had to say it for others.)

 

[Marbles]

Even canted upwards, gravity takes over the second it leaves the barrel and it starts dropping from the line of the bore.
(I know you know this but I had to say it for others.)

What if it is canted to be parallel to earths gravitational force..... Would the gravity from the sun or the moon be more significant or would both be insignificant compared to earths rotation? Sorry.

By the way, I don't know the answer to that, but would appreciate if someone does.

I'm pretty sure most people are referencing line of sight, not bore line. So they think of the fact that the bullet starts out below line of sight, crosses above it at the point blank, then drops below it again at zero distance. Granted, some certainly do not realize that line of sight and bore line are not parallel.

 

[RS_Non-Res]

Aye @Tnc, but normally the bullet it is rising for a little while towards the line of sight...But nobody ever states their frame of reference these days.

Maybe a different way to think about it....With cant, the vertically oriented 2-D plane in which the bullet travels, the trajectory, (neglecting wind) intersects the line of sight (LOS) (and the vertical plane through it) at only one range. Closer in the bullet is on one side of the LOS, beyond that range the bullet will be on the other side of the LOS. (@Marbles second envelope picture is a view from above).

When the system is aligned without any cant, plumb with gravity, the 2-D plane/trajectory intersects the LOS at all ranges. This is handy.

If you want the problem of intersecting planes to go away, and you want to keep your cant, you make sure that POI is offset (the right direction) from POA by the horizontal part of the height over bore measurement.

That was fun.

The moon's gravity is biggerer b/c it moves whole oceans.
Wait, the Sun moves whole planets.
IDK

edited for typos and other crud.

 

[Bert01]

15 degree cant with 2.25 inch scope height

Sin(15)x2.25in=0.58in offset from bore.

100 yard zero, on the horizontal axis you have a 0.58 shooters MOA angel. At 200 yards your shot will be off by 0.58 shooters MOA, it will still be off by 0.58 shooters MOA at 500 yards, but that is now a 2.33 inch error, at 1000 yards that is a 5.24 inch error. You could dial it out by offsetting your zero at 100 yards by 1/2 MOA.

Zero at 200 yards and the error becomes 0.29 shooters MOA. Zero at 300 and it becomes 0.19 shooters MOA.

In hunting, the offset at 15 degrees of cant is meaningless. However, why the hell anyone would want to shoot a rifle canted at 15 degrees is beyond me.

A 5 degree cant with 2.25 inch scope hight gives a 0.2 inch offset, so less than 1/4 MOA with a 100 yard zero, meaning it is meaningless.

45 degree cant with 2.25 scope hight is 1.59 inches of offset, with a 100 yard zero, you will be missing by 1.59 inches at 200 yards, more than is acceptable for sure, but not what I would call a ton.

The math is pretty simple, just working with trig.

Now, if the reticle is canted things get more complicated.

Trig problem number 1View attachment 782671

Trig problem number 2
View attachment 782672

Thanks @Marbles for drawing and writing it out. Thought I was going to have to do it.

The fact is it’s there mathematically, it may not matter but it’s there.

This is the same argument for sighting in a double rifle, whether to have the bullets converge at some set distance, cross and therefore diverge more as distance increases. Or to have both barrels shoot parallel, each one being a little left or right of the POA, but the same no matter the range.