Intereting article on scope height and scope size. It makes a lot of sense, the higher your scope, the more variables for error. Canting your scope left or right is also a greater possibility when your scope is mounted with high rings or you have a large objective lense that requires higher mounting. Read a detailed article describing this phenomenom.
SCOPE HEIGHT
by John Barsness
EVER SINCE THE 1960’s, when the original Redfield company started selling the first "practical" variable-magnification telescopic sights in the U.S., riflescopes have been getting bigger. These days it’s not uncommon to see scopes topping out at 20x or more on deer rifles. These "Hubbles" violate one of the old rules of scope mounting, still obsessed over by some hunters: Hunting scopes should be mounted as low as possible over the bore.
This is an interesting conflict, since it brings up all sorts of side-effects.
First, let’s look at the old notion that scopes should be mounted really low. This arose back in the days when most hunting rifles were designed for iron sights, with a relatively low-angling buttstock allowing the shooter’s eye to line up with the sights.
Also, in the early days of scopes, many had relatively small fields of view. This may seem odd, since most early hunting scopes were low-power, but they also had thin tubes of 7/8" or even ¾", and very small ocular lenses. So it helped to mount the scopes as close as possible to the same height as iron sights, since this helped the shooter "find" the small field of view.
Low mounting also had another benefit. Early scopes weren’t all that strong, thanks in part to the thin tubes. Scopes close to the rifle’s action weren’t as likely to be banged around and knocked out of alignment, and lower mounts tended to be stronger than higher mounts.
Back then most higher mounts were designed to allow the shooter an instant choice between iron sights and scope. Scopes were delicate, and also tended to fog, because they weren’t sealed against atmospheric moisture. But high mounts, whether inexpensive Holden Ironsighters or expensive Griffin & Howe side-mounts, allowed scopes to be knocked out of alignment more easily.
If you don’t believe this, find a rifle with either of those high mounts, put a collimator in the muzzle, and look through the scope while pressing on the tube from various directions. The scope’s reticle will wobble around amazingly, and may not even return to its original spot. If mere hand-pressure can do that, imagine what a real "hunting bump" might do. That’s another reason many experienced hunters preferred low-mounted scopes.
These days both scopes and mounts are tougher, and some (but not all) rifle stocks are designed to put our eye in line with a scope. With many rifles and scopes, low mounts can’t be used, because the objective bell of the scope won’t allow it. Modern scopes and stocks enable most hunters to instantly find a full field-of-view with their scope mounted in medium-height rings.
So there’s no real reason to mount scopes as low as possible over the bore. Or is there? One problem that can occur as scopes get mounted higher is the effect of "canting" is increased. Canting is aiming the rifle slightly tilted to one side or the other. Exactly how this causes problems takes some explaining.
Let’s say we sight-in a typical .30-06 so 180-grain bullets are landing in a group 2" directly above the point of aim at 100 yards. When doing this, we’re essentially manipulating a long, narrow X. One leg of the X is the line-of-sight of the scope’s reticle, and the other is the bullet’s flight. The bullet starts out below the scope, but at 100 yards is above the scope, so somewhere between the muzzle and 100 yards, the two legs of the X cross.
The line-of-sight is straight, but the bullet’s flight is slightly curved, because gravity starts pulling the bullet downward as soon as it leaves the muzzle. At 200 yards the bullet’s curve intersects straight line-of-sight of the reticle. Beyond 200 the bullet falls increasingly below the reticle line-of-sight.
Now, visualize aiming the same rifle as it lies on its left side. The pull of gravity has changed 90 degrees. The bullet still travels in the same curve, but the curve no longer aligns vertically with the reticle. Instead it curves below the reticle’s line of sight. The line-of-sight of the scope will still be straight, but instead of the bullet landing 2" above the reticle, it will land to the left of the reticle–and it will land a lot further than 2" left, and low.
To a smaller extent, this is exactly what happens we cant the rifle just a little. A bit of cant doesn’t cause enough change in point-of-impact out to 100 yards to make any significant difference on a deer or elk, but it can at longer ranges. And the effect of cant grows greater with the height of the scope above the bore.
Most of us can keep a rifle fairly level when shooting, but some, uh, can’t. The first time I encountered this was many years ago, when a local friend named Jim bought a new rifle and scope. He’d started making enough money to afford to go on guided hunts for various North American animals, and also decided he needed some lighter, more powerful rifle than the Ruger 77 .257 Roberts he’d been shooting for 30-some years. One was going to be a real mountain rifle for hunting Dall and Stone sheep.
So he bought a Weatherby Ultra Lightweight 7mm-08, and also decided he needed a more powerful scope than the old 3-9x Redfield on his .257. He bought a 4-16x with a 50mm adjustable objective. With this new high-tech rifle and scope, he fully expected to shoot groups even smaller than with his old .257. Instead the 7mm-08’s groups averaged 2-3", no matter what handloading combinations he tried.
Finally he asked me to help, so I took the 7mm-08 went to the range one day with Jim’s handloads, and some of mine. I shot a number of groups of 1" or less, so invited Jim to come along during the next range session. He sat down and proceeded to shoot a 2-1/2" group. We let the barrel cool, then he shot some more at the same target. Some bullets landed right where they should, but others landed to the right or left, and usually a little lower.
Now, I had seen Jim consistently shoot sub-inch groups with his .257, so his shooting ability wasn’t the problem. But the big scope was sitting way up there. I suggested he get a smaller scope and mount it lower. He had just been converted to the notion of a "Hubble" by a salesman in the local gun store, but agreed pretty readily to my suggestion, as he’d never really felt comfortable with the big scope. He traded the 4-16x back to the same store for a 3-9x and some medium rings. The right and left fliers went away, and Jim went on the kill two expensive wild sheep with the little rifle, along with a mountain caribou.
Some shooters apparently don’t have much sense of "level" when shooting, even on a level range, and Jim was one. This is the reason that many (if not most) long-range target shooters, and even some varmint shooters, often employ a scope level.
On the other hand, higher mounts do apparently flatten the trajectory of any rifle somewhat. Remember the two legs of the X of the scope reticle and the bullet? If we use higher scope mounts, the distance between the scope and muzzle becomes greater, so the bullet’s path angles slightly higher beyond 100 yards.
Let’s compare the trajectories of the same .30-06 and 180-grain load, sighted-in 2" high at 100 yards, with two different scope heights, using Sierra’s Infinity program.
First let’s calculate the trajectory with the scope 1.5" over the bore, an average height these days:
| 100 yards: | +2" |
| 200 yards: | -0- |
| 300 yards: | -8 |
| 400 yards: | -23.5 |
| 500 yards: | -47 |
Now let’s see what the same load does with the scope 2" over the bore:
| 100 yards: | +2" |
| 200 yards: | +.5 |
| 300 yards: | -7 |
| 400 yards: | -22 |
| 500 yards: | -45 |
Now, the trajectory really isn’t flatter, it’s just been manipulated by scope height. Is an inch at 300 yards or two inches at 500 enough gain to make the higher mounts worthwhile, especially since they increase the effects of cant?
Of course, these days real long-range shooters tend to click the elevation turret, so a slightly flatter trajectory makes even less difference. But clicking doesn’t change the effects of canting. So in at least one way it still makes sense to mount the scope as low as possible over the bore-even if the scopes we use these days can’t go as low as the Lyman Alaskan on grandpa’s Savage 99.
original article here: http://www.24hourcampfire.com/newsletters/February_2011.html