Home Forums Hunting & Shooting Ask The Gunwriters Ask John Barsness Questions About "SCOPE HEIGHT."

Just remember to give me credit in the article as usual. I will get even in the gopher fields in a few weeks...

John, in the past you wrote an article about stock fit and in it you wrote about a .375 H&H that the stock was fitted to you so when you brought it up the rifle lined up like a shotgun for you. I've always heard & read to mount scopes as low as possible. Now my scopes are mounted so when I mount my rifles I'm looking through the scope instead of scrunching my cheek down on the comb to get a proper alignment.

Thanks to your info I have my scopes mounted so my rifles are more comfortable to shoot and can get on target faster.

Dave,

Thanks! Good to hear.

Thought you were only allowed to use finger paint? !

Jeff

To this point in the discussion, it seems everyone is talking about the first point at which the bullet crosses line of sight(LOS).
Zeroed at 25 yards is zeroed at 25 yards no matter what the orientation of the rifle is.

My question is, "How does the steeper downward angle (relative to the bore/trajectory, not the earth) of the higher scope affect the trajectory compared to LOS once it passes the first zero point?"

And that is the pertinent question.

Maestro mds, the Hogema paper you linked to _appears_ to indicate that the avg. radii of the circles found experimentally varies between Low LOS and High LOS. Is that correct (statistically significantly)?

Please have a look at this image from there:

It seems to me that the POI's are on the same circle with either scope height.

I have shot rifle canting tests already (air rifle, 21 meters) and I can tell you that there is much more wobble than usual because you can't shoulder the rifle correctly, this is the reason of the relative big size of the groups.

If you mark the grouping size for the 0 degree shots and allow at least the same grouping for the canted shots as well, you'll find that they almost all are on the circle within this tolerance.

Or please have a look at my last shots: http://www.szottesfold.co.uk/2012/02/canting-error-teszteles.html
Sorry it's in my mother language but the pictures speak for themselves. Or you can use Google Translator from Hungarian to English.

That test has been done only with one scope height, previously I calculated the 'd' drop value from velocity and real BC (scope height was NOT involved) and drew the circle with radius 'd' on the paper, the hits landed of course along the circle.

Excellent work and graphics, maestro.

Maestro you're quite a contortionist to fire your pellet rifle inverted! Nice job demonstrating your point.

Now that it's shown theoretically and experimentally that the point of impact with reference to the first line of sight crossing (near point zero) doesn't change with cant, we need to test what happens after the projectile passes that near point zero. In other words, leave the rifle sighted for 21 meters and then fire it at 40 or 50 meters (or whatever limit a reliable group size can be maintained) with two different scope heights and different cant angles.

I concur that there won't be any deflection as long as one is shooting at the range at which it is zeroed but my thoughts are that once we shoot past that range that there will be more lateral deflection on targets beyond the near point zero.

If scope height didn't matter then it wouldn't have to be considered in normal trajectory calculations. I just ran an example on my I-phone ballistic calculator ("Ballistic") using a bullet with a BC of .500 at a velocity of 3000 fps on a standard day. With a 50 yard zero and a sight height of 0.5" the drop at 500 yds would be 51.9". With a 50 yard zero and a sight height of 3.0" the drop at 500 yards would be 29.4.

Now, granted that trajectories aren't straight lines, but let's assume that a rifle with the above parameters (zeroed at 50 yards but aimed at a target at 500 yards) is fired on its side and the bullet path is viewed from straight above, I'm thinking the L-R difference at 500 yards would be at least 13" or the difference of the two figures in the above paragraph (51.9" -29.4").

What do you think?

That's my guess as well, but will run my own tests--with a very accurate .30-06.

Maestro, I was referring primarily to the paper you linked, in which the table indicates a different radius for the Low LOS circle vs the High LOS circle (assuming "r" indicates the radius of the circle, as opposed to something else). The drawings in that paper also visually show differing diameters, though I was struggling a little to reconcile what I thought I saw in the diagrams vs what I thought I read in the table.
Different radii would indicate that, as you show, sight height is of little consequence at short range (such as air rifle range), but it would become a huge problem at longer range (such as centerfire rifle range).
So, I appear to still need help making sure I'm reading the publication correctly.

I think that this nothing, 1.7 mm difference while the groups were 8-9 mm big...

The author came to the same conclusion, anyway: "As Table I shows, no statistically significant difference was found between the low and high LOS condition for any of the circle parameters."

I'll write a detailed explanation tomorrow about your questions but first please help me with an information:

If you zero your rifle at 100 yds and want to shoot at 200 yds, what do you do?
- adjust the turrets by certain clicks?
- hold over by certain mildots?
- hold over by certain inches?
I mean not only you personally but in general the big bore rifle shooters, which method(s) do you use in everyday's life?

Thank you

I normally use holdover if the target is beyond my point blank range. In other words if I know my target is at 200 yds and I know the drop from a 100 yd zero is 4", I'll holdover what I estimate to be 4".

On the other hand, I do use some holdover type reticles on some rifles (e.g.-Leupold's B&C).

Some good discussion here.

With my '06 shooting 180gr bullets (and similar cartridges) I use the 2-0-8-24 method. Those numbers represent the bullet's path at 100, 200, 300, and 400 yards. Then I use the relative size of the animal to estimate holdover. A deer is about 16 inches thick from top of shoulders to brisket (18 inches for a big one). With the bullet dropping 8 inches at 300 yards, put the horizontal crosswire on the top of the deer. At 400 yards I pull the crosswire above the shoulder about the thickness of the deer's body for a total holdover of 24 inches. I don't shoot beyond that at live animals. This system works for me.

Windage is the real blinger. I tend to overestimate wind speed. Ten mph is a harder blow than most people realize, I think. Even if we accurately measure wind speed where we are standing, that says little about the wind at the target and everywhere in between. Honestly, I don't have a system for estimating wind drift, and I'm open to suggestions.

The scope's height above the bore is meaningful as regards trajectory. The effect of canting of the scope or the rifle is dependent on the degree of cant and how it is handled.
If, for instance, the scope is 2 inches above the bore, the rifle is canted 45 degrees left, and the scope is mounted so that the crosshairs are vertical, the scope will be offset by roughly 1.1 inches and the effective height will be about 1.1 inches as well. If this offset is ignored and the rifle is sighted so that is hits point of aim at 100 yds, it will then hit 1.1 inches to the left at 200 and will continue to diverge at the rate until it would be roughly ten inches left at 1000 yds. If the rifle was sighted so that the bullet hit 1/1 inches to the right at 100 yds, it would travel parallel to the line of sight (laterally, that is)and would remain 1.1 inches to the right from then on.
Of course, 45 degrees of cant is an extreme example but serves to illustrate.
There are two reason for mounting scopes higher IMO. The first is to accomodate the scope (large bell, etc.). The second reason is because the stock is straight enough that it becomes awkward for the shooter to get his head down. The AR15 platform is an example of a high line-of-sight mandated by a very straight and high stock design. Many of the old schuetzen-style rifles are examples of low sighting planes mandating a stock with a lot of drop.
So, gunmakers are, as always trapped in a world of compromise. They can make straight stocks which help to manage muzzle jump and look pretty good while doing so but they will have to settle with a scope which is mounted a bit higher than they might like (awkward appearance. exposed to damage).
I personally think stocks should have a little more drop at the heel than has become popular and svope should be mounted low. I like monte carlo combs too. This may reflect my age more than it reflects any reality but there it is! GD

You may be right, although a 15 - 20% difference failing to be statistically significant suggests that the test is either not very repeatable, the rifle (or shooter) is not very repeatable, too little testing was done to give much confidence, or the confidence interval used in the analysis is ...very different... from what one might more routinely expect to use.

Greydog's illustration is the idea I was coming around to, about how rifle cant could become significant at range. Nevertheless, his illustration also indicates how _insignificant_ rifle cant is in practical, hunting applications.

Sorry about the late answer fellow, but I wanted to write it thoroughly, and this has been a long story...

First about my theory:



I think this is all clear and logical. Let me mention that the 'd' drop value has nothing to do with scope height, bullet drop is only about the bore and projectile. Ballistic softwares need the scope height only for the trajectory calculations but not for the drop value. Drop at a given distance depends only on the muzzle velocity and the real BC value (which can be measured under the given environmental conditions).

About zeroing, this was an interesting question and lead me to some interesting conclusion...

First we have to define what 'zeroing' is. I assume that if you talk about zeroing then you mean that your rifle shoots at a certain X and Y distance (1st & 2nd zero) exactly where the crosshair looks and you hold over/under for the other distances. And when you talk about line of sight then you mean the line where the crosshair centre looks.

These concepts are common in shooting scene but I would rather say that zero is where we aim and where we want to hit, so the rifle is zeroed at a given distance when I know how to look through the scope in order to hit the target.

Anyhow, the thing is that if you use an other aim point, say a mildot, you use an other LOS which looks exactly to the target. The LOS is always pointing at the target, this is the trick. You adjust the line-of-sight (when shooting with clicks) or select from many pre-defined lines (when shooting with mildots). And then the bore line has to look with 'd' above so the bullet drops into the target etc.



So we know already that if we shoot with clicks or mildots (i.e. we do the required correction of LOS with our scope), the canting error is totally independent from the scope height.

* * * * * * * * * * * * * * * * * * * * * *

But now comes the twist! You have to know that I'm shooting FT and we usually align the LOS to the actual target with clicks or a few people use mildots, but whichever way, our LOS looks at the target and if we cant, we rotate the bore line around the target itself, see above.

I considered that all of these 'high scope is bad' opinions come from firearm shooters. Maybe they do anything other way which can change the things? And yes, they do.

I had to revise my stand. Still standing by all written above, I examined the third aiming method when one shoots to another aim point with the appropriate distance above the target.



The main difference is that when canting the rifle, the rotating axis of the bore line is not at the target but at the aiming point (e.g. the back of the ram). And in this case, the higher scope causes a bigger canting error, indeed.

So if we shoot with holdover measured at the target (i.e. we do the correction outside of the scope), the canting error grows together with the scope height (assumed that all other parameters are the same).

John, now I'm gonna tell you the result of your experiment already: if you aim with clicks or mildots at the two ranges then you can't get more difference than the grouping size and wind effects. But if you measure the hold-over on the target then the higher scope will give more canting error. Funny but both results would have been correct, depending on your shooting style.

I believed that the question is that the higher scope DOES or DOESN'T result a bigger canting error but the real answer is that it MAY, with certain aiming techniques.

Thanks for the cooperation my friends :-)