In precision shooting there are many factors that affect the ability of a shooter to hit a target with accuracy. One of these factors is cant error. Cant error is the result of not holding the rifle bore axis and the scope axis in a vertical plane.
"I simply cannot imagine a long-range aficionado putting up with it." - Jim See
When a shooter sights a target through a scope, they’re looking along a straight line that isn’t affected by wind, gravity, bullet velocity, or any of the various factors that normally affect a bullet in flight.
To compensate for real world factors, such as gravity, modern scopes have adjustments to raise the tip of the gun barrel so the bullet reaches the target. After making this adjustment, the gun barrel is no longer parallel to the straight line of sight from your eye to the target.
This is where cant error comes into play. Once you’ve raise the tip of the barrel so the barrel isn’t parallel with the line of sight, if the firearm is tilted around the line of sight (tilted either left or right) you create “cant error”. The effect of cant error is the bullet will impact the target to the side (the side you tilted toward) and low of your desired impact point.
As with all shooting errors, how much you miss the desired impact point by is proportional to how far away the target is. As technology is improving, and allowing accurate shooting at greater and greater distances, cant error is an ever increasing factor.
Cant error can be eliminated by ensuring the scope’s horizontal cross hair is level, or horizontal. Scope levels are the tool of choice for determining horizontal. Many shooters use the horizon as a horizontal reference, however unless the horizon is a large body of water, it’s likely the horizon that appears where the ground meets the sky, is actually horizontal. Sloping terrain creates a false horizon, which leads once again to cant error.
A good scope level is the most efficient and accurate way to eliminate cant error. It ensures the rifle is always held in the same attitude as when you sighted it in at the range.
If you have trouble picturing cant error, try thinking of it in the most extreme case. Imagine looking through your scope at a target 300 yards away. Your scope has compensated for the trajectory by lifting the tip of your barrel. Now the line through the center of the barrel is not parallel to the line of sight through the scope.
Now imagine rotating your rifle 90 degrees around the line of sight. This makes the barrel point off to the side instead of upward. Obviously the bullet will travel to the side of its intended target.
Since the barrel is not raised now to compensate for gravity, the bullet will also impact the target below the desired point.
Of course you’d never rotate your rifle 90 degrees, but any amount of rotation causes cant error, and the more you’re off center, the more cant error you have.