Many of us have experienced the frustration of missing the X ring even when all conditions and the shot release appeared to be perfect.
"What else is new", you may say. Well, not all misses are your fault or due to misjudging the wind. Some may be due to a small unseen and unexpected equipment problem.
The misses I'm talking about are less common in situations where a front rest with levelling adjustments is used as in benchrest shooting and very much more common when a soft bag front rest or no front rest at all is used.
This may, in part be due to something called "CANT".
Cant is essentially holding the rifle, at the time it is fired, (and thus the sights), off the vertical plane in which it was zeroed.
Scopes and other sights are zeroed so that the crosshairs are in true vertical and horizontal planes. You do this so that elevation adjustments move truly vertically and so that windage adjustments move truly horizontally. In other words, it is important for accuracy, that in trying to move your point of impact vertically, the cross hairs move only up or down and not left and right at the same time. The same would be true of a windage adjustment where you want the cross hairs to move horizontally and not at all vertically.
A scope is mounted an inch or more above the bore of a rifle and so it cannot be in the same plane as the bore of the rifle. If the rifle is canted slightly to one side, it may rotate around the bore but not around the bore and the scope at the same time. When the rifle is canted, and you are ready to fire, you place the sights on your point of aim but your rifle is canted and so the point of impact and the point of aim are no longer aligned. What happens is that canting the rifle changes the point of impact shown in the cross hairs. While the rifle may appear to be perfectly zeroed, the cross hairs may actually point to a spot above or below, and left or right, of the actual point of impact. In good, lower light conditions, you might be able to see this with a laser bore sighter at shorter ranges.
The effect is noticable in short range shooting but in long range shooting, the effect is multiplied and dramatic. For example, at 1000yds, a .30 caliber bullet, fired from a rifle canted only 6 degrees will miss the point of aim by about 55 inches. Even on a short range the same 6 degree cant at 100 yds can result in a 1/2 inch miss. Double everything for a 12 degree cant. These numbers would change depending on the caliber, the distance you are shooting, the characteristics of the bullet, the height of the scope above the bore and other factors.
Six degrees is equivalent to one minute on the face of a clock. This is so small that without reference lines, an average person/shooter cannot see the difference between a vertical hold and a canted hold.
There is a relatively inexpensive way to provide a reference and to correct for cant. Try a properly installed levelling bubble in an anti-cant accessory which allows you to bring the rifle to a true vertical hold identical to what you did when zeroing. Level the bubble before you shoot and you have elliminated errors due to cant.
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