fact or fiction: higher scope height increases cant error (and how much)

[Macintosh]

I am sure this has been discussed, but seems to have some myth surrounding it, so figured I'd start a new thread to ask specifically. I can find a number of articles on the topic of cant, but few that quantify the error including scope height as a variable. rifle cant causes error at long range--that is not in question. The question is whether a HIGHER scope mount on the same rifle will induce more, less or the same error at long range.

I made a comment in another thread to the effect of "higher scope height increases the error from cant". In response, another poster @JohnJohnson posted an MDT video where they tested this.

The recent discussion was posts 881-891 here: https://rokslide.com/forums/threads/maven-rs1-2-2-5-15x44-new-model.333202/page-45#post-3439453

The MDT video:

The article I posted--note this article is DEBUNKING the "myth" that a higher scope causes more cant error--however, he cites an EXCEPTION to this rule for a rifle with a zero distance that is less than the distance being tested. Apparently he is coming at this from the perspective of fixed, known-distance target shooting, and a situation like having a 100 or 200 yard zero shooting at a target that is at 900 yards is an "exception"... so his conclusion is that scope height doesnt matter, but its not clear to me that he isnt discounting the "exception", which may in fact be more relevant for our purposes. The relevant section of the long and detailed post is screenshot below as well. http://www.szottesfold.co.uk/2012/03/high-scope-and-canting-end-of-ancient.html



from the article:

PART IV. – CONCLUSIONS

1. In general...

2. If we do the correction outside the scope (i.e. we have zeroed the rifle at a given distance and then shoot at another target with a holdover which is measured at the target), the canting error can be different – more or less – with the higher scopes. It will be more if the new range is greater than the zero range and less if it's shorter.
....

So, it seems clear from his conclusion that in a situation where you are shooting at a range greater than your zero-distance, he is saying a higher scope WILL increase cant error (bolded section of his conclusion above, as well as his chapter 3).

My questions:

1) are the mdt video and the article at odds? If so, which is correct?
2) what is the FUNCTIONAL difference on target at ranges we shoot? ie if we are talking about a scope with a 1.5" mounting height versus one with a 2.25" mounting height (or whatever realistic min/max is for the sort of hunting rifles folks here are using), zeroed at 100 yards, what is the actual increase in error on target at 500, 600, 800, 1000 yards when we introduce cant error? I dont have the mathing ability to calculate this and could not find any quantified info on the subject beyond what was already referenced.

Edit: I emailed the guy who wrote this article and his reply when I asked to clarify was that if you use a dialed solution, or a reticle-hold, it is the same--in both cases the scope height does not affect the amount of error from cant. He said what he meant was that if you leave your zero as-is and aim using the center crosshair, but aim AT a point on the target above where you want bullet to impact (i.e. you aim at the top of an animal's back or a point above this as a "holdover"), THAT is when a higher scope would amplify the error from any cant.

 

[JohnJohnson]

Apparently he is coming at this from the perspective of fixed, known-distance target shooting, and a situation like having a 100 or 200 yard zero shooting at a target that is at 900 yards is an "exception"... so his conclusion is that scope height doesnt matter, but its not clear to me that he isnt discounting the "exception", which may in fact be more relevant for our purposes.

Unless I'm misinterpreting what he's saying, "zero distance" is what your scope is dialed for. So if I dial elevation for 900 yards, then 900 yards is my zero-distance. What he's saying about the exception is only for when your scope is zeroed for one distance and you hold over using the reticle to solve the elevation. If you dial elevation, it becomes a non-issue. The MDT video and the article are not at odds because in the MDT video they are "zeroed" for the distance they are testing at, and not using the reticle to hold over for it.

 

[Macintosh]

OK, makes sense--I think. Let me check your understanding of this-- basically you are saying that dialing the solution (5.6mils, or whatever) is the equivalant of being zeroed at that range, and once dialed scope height differences no longer add to the cant error (ie the error is still there, but the amount of error is the same regardless of whether it is a low or high mount)...BUT, if you leave your scope zeroed at 100 or 200 yards, and use a reticle holdover (ie rather than dialing you hold on the 5.5mil hashmark) the error WILL increase as scope height increases?

 

[JohnJohnson]

OK, makes sense--I think. Let me check your understanding of this-- basically you are saying that dialing the solution (5.6mils, or whatever) is the equivalant of being zeroed at that range, and once dialed scope height differences no longer add to the cant error (ie the error is still there, but the amount of error is the same regardless of whether it is a low or high mount)...BUT, if you leave your scope zeroed at 100 or 200 yards, and use a reticle holdover (ie rather than dialing you hold on the 5.5mil hashmark) the error WILL increase as scope height increases?

That is what the article you linked is asserting, yes. I'll leave it to someone who's a hell of a lot better at math than I am to verify his math is correct. But it does match up with MDT's testing in the video. I have yet to see any evidence to the contrary. And I still have no idea to what degree the extra error changes things for holdovers, and whether it's a big enough change that there's a practical difference.

 

[Macintosh]

Copy. I cant say it makes intuitive sense to me one way or another…actually maybe that even holdovers should still be irrelevant if we are canting around the bore and our line of sight (the 5.5mil hash mark) is the new “zero” intersecting with the bore-line at the target. Anyone here a mega trig nerd and care to take a stab at quantifying this?

 

[JohnJohnson]

Copy. Anyone here a mega trig nerd and care to take a stab at quantifying this?

I think the only loose end is quantifying the error from 1.5-2.5" height over bore for reticle elevations holdovers assuming a 100 yard zero. I'm towards the extreme end of height over bore, mine is 2.5" with ARC 1.42" rings on my Gen 3 Razor. But it'll be good to have this thread anyway since I see people say it a lot, even very experienced shooters.

To prevent people from having to read the replies if this picks up any steam, maybe you could add in a section to the end of the original post mentioning what we concluded so far?

 

[Macintosh]

Yep— the diagram linked (and trashed) in the article I linked is from the Accurate Shooter article on cant, which I think (?) is generally viewed as a solid source of info on the topic.

 

[hereinaz]

I think he is saying that the cant error is more with holdovers, but the height of the scope still doesn't affect the magnitude of the error.

I'll have to read it again to confirm.

 

[Macintosh]

I actually emailed the guy who wrote the article I linked to and asked him if he could clarify. His reply was that if you use a dialed solution, or a reticle-hold, it is the same--in both cases the scope height does not affect the amount of error from cant. He said what he meant was that if you leave your zero as-is and aim using the center crosshair, but aim AT a point on the target above where you want bullet to impact (i.e. you aim at the top of an animal's back or a point above this as a "holdover"), THAT is when a higher scope would amplify the error from any cant.

 

[JohnJohnson]

He said what he meant was that if you leave your zero as-is and aim using the center crosshair, but aim AT a point on the target above where you want bullet to impact (i.e. you aim at the top of an animal's back or a point above this as a "holdover"), THAT is when a higher scope would amplify the error from any cant.

I don't quite understand how that would not be the same as a reticle holdover but it's good to know that really either way there's no effect from height over bore. Now I can just link people this thread whenever someone repeats the myth.

 

[Macintosh]

yeah, I cant say I have a feel for what the difference is, but that's what he said. I asked about quantifying it and will post if he replies, but hopefully if anyone has a sense of the detail on this they can chime in. I feel like I'm a bit over my head with the mathing at this point!

 

[Shortschaf]

1) are the mdt video and the article at odds? If so, which is correct?
2) what is the FUNCTIONAL difference on target at ranges we shoot?

1) The two sources are not at odds and in fact agree. MDT is shows that height-over-bore doesn't affect accuracy/capability but assumes the system is NOT CANTED. The article agrees, it just goes a bit further in chapter 3 which assumes the system IS CANTED.

2) Here's the comparison of a scope that is perfectly aligned to the bore and one that is THIRTY SIX inches above the bore.






Equation for someone to check my math
=SIN((cant angle in degrees)*3.141592654/180)*(Elevation in MILs to dial 1000 yards)*36+SIN((cant angle in degrees)*3.141592654/180)*(scope height above bore)

 

[180ls1]

1) The two sources are not at odds and in fact agree. MDT is shows that height-over-bore doesn't affect accuracy/capability but assumes the system is NOT CANTED. The article agrees, it just goes a bit further in chapter 3 which assumes the system IS CANTED.

2) Here's the comparison of a scope that is perfectly aligned to the bore and one that is THIRTY SIX inches above the bore.


View attachment 655314



Equation for someone to check my math
=TAN((cant angle in degrees)*3.141592654/180)*(Elevation in MILs to dial 1000 yards)*36+TAN((cant angle in degrees)*3.141592654/180)*(scope height above bore)

So, yes, it does technically speaking. However, for hunters, it's so minuscule given the mounting height options we use it's not worth a second thought.

Yeah?

 

[JohnJohnson]

So, yes, it does technically speaking. However, for hunters, it's so minuscule given the mounting height options we use it's not worth a second thought.

Yeah?

I'd take that further and say that for literally anyone not using over 5" (lol) height over bore it has no effect at all. The cant error for 0.5-5 degrees of cant for the 1" and 2" are identical.

Even the 36" high scope base (with included system cant too) only has an extra 0.2 MIL of cant error at 5 degrees at 1000 yards compared to the 1" and 2" height. So for any actual gun and actual height over bore the difference is so small it's non-existent for practical purposes.

 

[Macintosh]

so, assuming that is correct--I mean, it's super mathy-looking, so it must be, right?? --what is the difference between 1.5inches above bore, and 2.5inches above bore? I see 1" and 2" are the "same"--I assume that is a rounding to the nearest tenth issue, and that they are slightly different if we measured to the thousandth of an inch, soseems like it's inside the resolution of a .1mil or .25moa scope? And, are you saying this applies to turret adjustments? That would seem to contradict the article. But maybe if the difference is there, its just un-measurable within the resolution we care about, and therefore it's purely academic whether there is a difference or not?

 

[180ls1]

I'd take that further and say that for literally anyone not using over 5" (lol) height over bore it has no effect at all. The cant error for 0.5-5 degrees of cant for the 1" and 2" are identical.

Even the 36" high scope base only has an extra 0.2 MIL of cant error at 5 degrees at 1000 yards compared to the 1" and 2" height. So for any actual gun and actual height over bore the difference is so small it's non-existent for practical purposes.

Good to know. So, not even the match guys who shoot a mile or two?

 

[Shortschaf]

so, assuming that is correct--I mean, it's super mathy-looking, so it must be, right?? --

It is mathy, but I have been known to make poor assumptions or miss things before too. If something dawns on me, I'll update.

what is the difference between 1.5inches above bore, and 2.5inches above bore?

Difference between 1.5" and 2.5" will be effectively the same as 1" to 2" --- NIL. Since you asked, I expanded the decimals, and the difference is 0.02 inches at 1000 yards.

I see 1" and 2" are the "same"--I assume that is a rounding to the nearest tenth issue, and that they are slightly different if we measured to the thousandth of an inch, soseems like it's inside the resolution of a .1mil or .25moa scope?

I purposely didn't show the second decimal place because we have no way to account for it in the real world. OP was all about what is FUNCTIONAL. And aside from a thought exercise, there is zero functional difference between the two.

And, are you saying this applies to turret adjustments?

No. Any of those combinations of scope heights, zeros, and elevation holdovers/dials will get you on target at 1000 provided you don't cant your rifle.

That would seem to contradict the article. But maybe if the difference is there, its just un-measurable within the resolution we care about, and therefore it's purely academic whether there is a difference or not?

Purely academic

 

[Shortschaf]

Good to know. So, not even the match guys who shoot a mile or two?

Correct.

The BIGGEST TAKEAWAY this should have is that you should always have a level and always use your level. Becuase if you do that, you could mount your scope 100 feet above your rifle and it would still aim true

 

[hereinaz]

I actually emailed the guy who wrote the article I linked to and asked him if he could clarify. His reply was that if you use a dialed solution, or a reticle-hold, it is the same--in both cases the scope height does not affect the amount of error from cant. He said what he meant was that if you leave your zero as-is and aim using the center crosshair, but aim AT a point on the target above where you want bullet to impact (i.e. you aim at the top of an animal's back or a point above this as a "holdover"), THAT is when a higher scope would amplify the error from any cant.

That makes sense.

Higher rings have never bothered me because I use a level and I dial elevation when hunting. I learned long ago that rifle cant will cause misses when I was doing the local PRS matches. That stuff focuses the mind.

 

[Shortschaf]

UPDATE: I used the wrong trig function above and edited the table accordingly.

Due to the small angles that were being used (less than 5°), correcting the equation to use SIN instead of TAN was entirely academic. The calculated answers change less than 1% between the two. But felt it was necessary to clear that up.