In previous long range shooting articles, I’ve talked about the effect of light and the error derived from shooting uphill/downhill—both aiming and perspective related errors. In this article, however, I’ll talk about another aiming-related error: shooting with the rifle not perpendicular to the ground.
When shooting with a canted rifle, the round won’t land where you are aiming (POI doesn’t correspond to POA), instead, it travels in the direction of the cant, and a tad beneath the aimed point.
Here’s an example: using our usual .308Win, with a 175gr HPBT bullet fired with a muzzle velocity of 2700fps, if you shoot at 1000yds with the rifle canted only 5° to the left, the shot will fall 3.3” to the left. It will also wind up about ½in low, but, at that distance, the error’s more or less negligible.
The error increases for wider cant angles, and also slightly increases with greater line of sight height, that is, the scope mount height above the barrel’s center line.
Why does canting the rifle generates this aiming error?
When you aim for a given distance, you keep the line of sight pointed at the target . The line of departure, on the other hand, is not pointed to the target, nor is it parallel to the line of sight. To compensate for the force of gravity and the resulting bullet drop, the line of departure is always inclined toward the line of sight.
When you shoot with the rifle canted to one side, you actually keep the line of sight axis as a pivot point, while you rotate the line of departure underneath. Since the line of departure is at an angle with respect to the line of sight, if the line of departure is rotated, it will no longer be oriented on the same vertical line as the line of sight. Consequently, the bullet will no longer travel in the direction of the aimed point, but instead, it will fly toward the direction of the cant.
Also, canting the rifle to one side, the vertical angle between the line of departure and the line of sight decreases. A decrease in the angle between LoD and LoS, compensates less for the distance and the drop, which lowers the point of impact.
Unlike with the other variables and errors we’ve seen before, you don’t have to compensate for the cant error, you just have to avoid it. Generally, your eye is capable of keeping the scope’s reticle perpendicular enough to minimize the problem. However, if you need extreme accuracy and you want to be sure to avoid cant error, you can mount an anti-cant bubble level, so you have the ability to verify precisely how level your rifle position is. Another important thing to do to avoid cant error is to mount the scope with the reticle perpendicular to the rifle’s bore axis. Indeed, if the scope is mounted with some degree of cant, when you turn the elevation knob to compensate for drop, you actually don’t compensate vertically, following the trajectory, but at an angle. The result creates an aiming error and a cant error, provided you use the scope reticle as canting reference.
In the next article, I’ll start to talk about a very important topic, essential for the long range shooting and for precision rifle shooting in general: bullet stabilization.
Featured image, courtesy of LaRuetactical.com, depicts an example of an anti-cant device from US Optics.