Rifle Cant and Canting
By Terry Hart
Almost all of us tilt our gun when shooting and don't even know we are doing it. This tendency is called "cant," or canting, and has been recognized and understood since the very early days of firearms use.
The buffalo hunters on the frontier used to have levels built into the bottom of their rear sights to get around this problem. Canting, or tilting, your rifle moves and opens up your group. You may be setting there saying, "Yeah but I've never noticed it and it can't be that big of a deal". It is! And depending on how you and your gun fit each other we all unconsciously do it to some degree.
The formulas for calculating cant errors are quite complex. Some of the things that effect the amount of this error are how much we cant the rifle, how far above the bore the scope is mounted, the distance to the target, the trajectory is of the cartridge, and what range you zeroed at relative to the range at which you are shooting.
One might assume that if you always shoot at the same range you zeroed at then it would be safe to ignore the affect of cant. Even this is not true. If you shoot a group at any range while holding your gun perfectly vertical, and then another at that same range with the gun only slighted canted, the difference will be quite noticeable. For typical amounts of cant these errors will be on the order of 1 inch per 100 yards left or right, depending on which way you leaned, and also 1 inch low per 100 yards.
If you are aiming at a target at some range that is significantly different from the distance at which you zeroed your rifle, these errors will quickly grow to feet. Cant errors on the order of 5 to 10 feet at 1,000 yards are common.
The first thing you should do to get rid of these errors is to make sure that your scope cross hairs are set perfectly vertical above the bore. Most of us shoulder our gun several times when installing a scope and set the cross hairs so that they look correct.
The problem is that when we do this we will also be unknowingly canting whatever amount is typical for the fit of that rifle and end up setting the cross hairs so they look true with the gun tilted this unconscious amount.
This simple to correct problem is the largest single source of canting errors. Always clamp your gun into a level fixture and align the cross hairs with some distant known vertical or horizontal target.
Second, when shooting at some distant target in the field it is normal to look for some handy limb, tree, rock, etc for a steady support. Seldom are these natural features really level, and even the slightest angle will result in errors. It is important to look through the scope for some vertical or horizontal reference and to consciously level your gun.
This is far from a perfect method, but is better than nothing. There won't always be anything visible to use and in those cases you are going to get some of this error.
There are readily available inexpensive indicators, or bubbles, that attach to scopes to get around this problem. But since most of us shoot with one eye closed their effectiveness is limited because we can't look through the scope and at these devices at the same time. Even so they are better than nothing, do at least provide a reference, and constantly remind us of the importance of cant.
There are also a few scopes around that have built in level indicators that are ideal. Problem is, these are usually very expensive and not suited for hunting.
At least be aware of and understand the affect of cant. There are endless documents published and available on the Internet where you can read about this subject to your hearts content. How far you choose to go in dealing with it is, as always, strictly up to you.
Copyright 2005 by Terry Hart. All rights reserved.