TECH TIP: Keep Your Rifle Level for Better Scores
Experienced marksmen know they should keep their rifles level when shooting. But they may not understand exactly what happens if they allow their rifle to be canted (tilted left or right), even a few degrees. While the physics are complicated to explain, here’s what you need to know: if you cant your rifle to the left, your shots will impact to the left, and lower, than your point of aim. Likewise, if you cant your rifle to the right, your bullets will impact low and right.
Effects of Rifle Canting
The effects of rifle canting are explained in great detail on the Long Shot Products Ltd. website. There, you’ll find a technical discussion of the Physics of Rifle Canting, plus a page with Sample Targets shot with canted rifles.
Referring to the above illustration, the Long Shot Products article explains: “Notice how the trajectory of the vertical hold stays within the vertical plane, so when the projectile drops, it drops into the line of sight and down to the center of the target. The trajectory of the cant hold does not achieve the same height as the trajectory of the vertical hold and the projectile diverges from the line of sight, thereby missing the target.”
The Long-Shot article makes two other important points. First, cant error increases with distance, and second, cant-induced windage errors are worsened by mounting your scope high above the bore axis:
“This component of cant error becomes more significant at more distant targets due to the increased original included angle between the line of sight axis and the bore axis (more elevation compensation) at the vertical hold.”
“Use of large-diameter objective scopes, mounted high off the barrel, exacerbates the cant error problem. To keep the scope elevation knobs centered for maximum adjustment, precision shooters sometimes use elevation-compensated scope mounting rings or bases. Although this solves the adjustment problem, it greatly exaggerates cant error because the distance between the bore axis and the line of sight axis increases and the included angle between the sight axis and the bore is larger, producing more windage error when canting.”
Test Targets Reveal Cant Errors
The Long Shot Products Ltd. website also displays actual Test Targets showing the effects of canting error. These targets were shot with air rifles and rimfire rifles, but the same effects can and will occur with centerfire rifles. Shown below is a target shot at 50 yards with a Feinwerkbau .22LR match rifle using RWS Match ammo (1012 fps MV). As you can see, canting the rifle 20 degrees to the left produced a huge movement of the point of impact. The shots from the canted rifle impacted 1.81″ Left, and 0.6″ below the point of aim.
John Krieger is widely recognized as one of the wizards of barrel-making. 
