You Can Understand Cant
A slanted view of shooting geometry.
We get asked a lot: “What if I don’t get my optic mount perfectly level with the receiver?” or some version of that question. It’s a good question. The short answer is: it’s important to good looks and almost certainly irrelevant to function, but this article is about the long answer.
There is very good reason to be concerned with cant: There is always some angle between the barrel and your line of sight (sight angle) at least here on Earth, because of gravity. The bullet starts falling as soon as it leaves the muzzle. The barrel must therefore be pointed upward (elevated) so the bullet will rise, then arch over and fall squarely into the point of aim at the desired distance. The elevation is of course accomplished by adjusting the sight angle, either with an internal scope adjustment, an externally adjustable scope mount, or by raising the rear iron sight relative to the front sight.
Imagine you have the proper sight angle for a 300 yard shot on a calm day, and you then turn the rifle on its side. What do you have? Effectively, your elevation is now your windage (side-to-side angle). If you turned (canted) the rifle 90 degrees to the left, what was previously (for instance) a 6 minutes-of-angle elevation, is now 6 MOA of left windage, and you have no elevation at all. If you take aim at your target and fire, the bullet will go drastically to the left and fall drastically short of the target. In our mental exercise we zeroed the sight with one orientation and then fired it with another. This is where you need to be concerned with cant — it is consistency that counts in marksmanship.
Now say we zeroed the rifle while holding it 90 degrees sideways, so the scope is straight off to the left side of the rifle (using the windage knob to make elevation adjustments, etc.) and then fired the shot while holding it sideways. If we did everything right, the shot will be successful. But there’s a new issue — the bullet’s path will have to start at the muzzle, and then cross from right-to-left, converging with the line of sight at the Point of Aim (POA). Lets say the sight is 2 inches to the left of the barrel center in our sideways model. The 300 yard zero means that we hit dead on point of aim at 300 yards (we have a perfect rifle, perfect ammo, a perfect shooter, and perfectly consistent atmospheric and temperature conditions). But at the muzzle, we’ll hit about 2 inches to the right of aim, because the sight is 2 inches to the left side of the barrel. Going out to 600 yards, that would mean we have a point of impact that is about 2 inches to the left of POA (the bullet has moved two inches leftward to converge with line of sight at 300 yards, crossed over and continued its leftward travel another 300 yards farther). We therefore have a windage error of 2 inches right at the muzzle, becoming less of an error as the bullet converges with POA at the 300 yard zero point, then we get an increasing error that ends up being 2 inches left at 600 yards, and grows worse beyond 600. If our sideways rifle has a 100 yard zero, it will shoot 2 inches left at 200 yards, 4 inches at 300 yards, etc.
Keep in mind that this is a worst-case scenario. The sight is in effect mounted completely to the side of the barrel, rather than on top. Two inches at 600 yards equals about .33 MOA. How many shooters are going to find a .33 MOA error in their windage to be a nagging problem? Not me, I can tell you. I’d probably never notice, what with wind being a constantly changing factor, plus I and my rifle aren’t perfect either. The 100 yard zero, with its 4 inch error at 300, would start to be a problem for me, but if you understand that you have this error, which is easily predicted and compensated for at the various distances, you have no problem other than the need to think about it when shooting beyond 200 yards at small targets. (There are issues related to point blank range and point blank target size, which prefer a sight be mounted above the barrel, but you’ll have to get Modern Ballistics to find out about those).
In our last model we kept a perfectly consistent hold, with regard to the rotational position of the rifle, for each shot. Going back to the standard rifle configuration with the sight mounted on top the barrel: Say we have carefully zeroed the sight at 300 yards. Without changing anything else we now rotate (cant) the rifle a couple degrees to the left. Some of our elevation angle is now converted to windage, so we lose some elevation and gain some left wind. Our 300 yard shot is going to hit low and to the left even if everything else was done perfectly. It is for this reason that some long distance shooters use a bubble level on the rifle, and some optic manufacturers install a bubble inside the scope itself. Keeping the rifle at the same cant from shot to shot is critical to accurate shooting, and gets more critical with longer distances or slower cartridges (more sight angle).
But there’s more to the story. So far so good — you have your perfect rifle perfectly zeroed at 300 yards and you hold the rifle perfectly consistent for every shot. You have your bubble mounted on the receiver and your confidence is high. But now you want to take a 425 yard shot at a 5-inch target. You know your trajectory exactly, because you have and use a copy of Modern Ballistics, and you dial in some more elevation using the super precise target knobs on your 3,000 dollar scope, you’ve doped the wind to within 0.5 MPH, you take careful aim while paying close attention to the bubble level on your receiver, you squeeze the trigger for a clean, surprise break (beautiful) and you miss the shot terribly. This is because we haven’t addressed the scope’s orentation with regard to the direction of Earth’s gravitational force. In short, we haven’t leveled the scope, so its elevation and windage adjustments are not “pure”. This matters only in cases where you are changing the sight’s settings. When you dial in more wind, you want to get pure wind (no elevation change). When you dial in more or less elevation, you want pure elevation change, and no change in windage. to accomplish that, the sight has to be perfectly level as you hold the rifle. As we discussed earlier, it matters not at all whether the rifle’s receiver is “level” or whether the mount is “level” or whether the sight is positioned exactly above the barrel, so long as you hold it the same for each shot. What matters here is that the sight’s adjustment vectors are level, so your adjustments are pure. Otherwise, you can do everything else right and still miss whenever you change your sight settings.
When using a 1x reflex sight, where you might never change the settings, it doesn’t really matter — it may be a little more time consuming to get it zeroed because your elevation will change when you’re trying to set the windage, but you’ll get it zeroed eventually. It can then stay put and so long as your hold is consistent, your zero remains fixed. Because you’re not making regular changes in the settings, the adjustment vectors don’t matter.
Most scopes are leveled by rotating the scope in its rings, aligning the horizontal cross hair with something in the environment that you know to be level (or aligning the vertical cross hair with something you know to be plumb) while you look through the scope using your preferred hold on the rifle. A dot sight has no crosshairs to use as a reference, so most people will simply eyeball the outside of the sight and use the windage and elevation knobs as a reference. If you really wanted to be sure, you could mount the rifle in a vice so that it cannot move (“leveled” with your preferred hold) and look through the sight at a plumb line. Place the dot on the plumb line and crank the elevation knob through its range of adjustment. If the sight is properly leveled, the dot will move up and down exactly parallel to the plumb line. Any other method of leveling your sight, or calibrating your bubble, is based on assumptions. This method allows you to calibrate the actual adjustment vector to that of the pull of gravity, without relying on any assumptions.
Using your cant understanding
Most dealers will mount a scope so that its cross hairs look square with the receiver. It’s the right thing to do, not knowing the shooter, but that is mostly a cosmetic issue. It assumes that every shooter will prefer to hold the receiver square with gravity. Some will, some won’t. Since we’ve established that the scope’s adjustment vectors do need to be level, and for each shot, and that the side-to-side position of the scope on the rifle (within a few degrees at least) matters not at all, we have choices. For one thing, if you’re installing a scope mount that clamps to the barrel, such as the UltiMAK M8, or if you’re drilling and tapping your own mounting holes in a receiver, you now know that if you fail to get the mount perfectly level with the receiver, it makes no difference. You know what really matters is that the the sight itself is level as you hold the rifle — not that the mount is level with the top of the receiver or some other arbitrary surface. You also know that you level the sight by rotating it in its rings, without regard to the mount. If it is more comfortable to cant the top of the rifle toward you so as to get the sight in front of your eye more easily, it means you might be able to shoot a little bit better with some leftward cant. No problem. Hold the rifle canted comfortably toward you as you level the scope in its rings. There you have it — rifle canted, scope adjustments perfectly level. Adjust your bubble so that it indicates a level reticle, and you can use your canted setup with complete confidence. The key is consistency of hold. Some competition shooters will mount their iron sights in a goofy looking orientation for the same reason, but they’ll drill holes in their targets with marvelous consistency.