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January 16th, 2012

Effects of Rifle Cant at Long Range — Tubb Explains

Eleven-Time NRA National High Power Champion David Tubb knows a bit about long-range shooting. One of the key factors in long-range accuracy is making sure that the tilt/cant of your rifle does not change throughout your shot string. In the clip below, the first in McMillan’s Master Class Video series, David Tubb explains the importance of keeping your rifle level. He explains that, at 1000 yards, your Point of Impact can change dramatically by canting the rifle either right or left. David states that, when shooting at 1000 yards, if your rifle is level and your shot is centered-up on a 72″ (six-foot) square target, you can actually put your next shot OFF PAPER by canting your rifle. That means you can move Point of Impact (POI) three feet or more, just by canting your rifle! Bryan Litz confirms Tubb’s observation. Bryan tells us that, as a general rule of thumb (for common cartridges), a 1° cant will produce five (5) inches of lateral displacement at 1000 yards. Thus, if you cant your rifle just 8°, the POI would move 40″ from the center of the target, putting the shot off the edge of a 72″-wide target.

David explains that, after one of his students has made two or three 1000-yard, X-Ring hits with a LEVEL rifle, “then I’ll have him take his rifle, and cant it to the right. I’ll have him shoot a shot. He will MISS the six-foot-square frame off to the right. Then I’ll have him cant his gun to the left and shoot another shot. He will miss the six-foot-square frame to the left.”

Rifle Hold and Canting — Consistency Counts
bubble level mount Remember that you must maintain the exact same amount of rifle cant from shot to shot. Yes, some iron sights shooters do tilt their rifle slightly to achieve a better hold or to index their sights better. However, these shooters do not change the tilt from shot to shot — the amount of cant remains the same on every shot.

When shooting prone with a scoped rifle it’s probably best to keep the rifle dead level, with the scope’s vertical crosshair straight up and down. Use a rifle-mounted bubble level to maintain a level hold, and avoid canting the gun either to the left or to the right. Affordable bubble levels that mount to your scope or scope rail can be purchased from Brownells and other vendors, starting at about $15.00.

Permalink - Videos, Competition, Shooting Skills 6 Comments »

June 28th, 2009

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.