The following test was designed to illustrate the results of canting a scoped rifle. Targets were shot with airgun, smallbore, and highpower rifles at preprinted targets designed to help the shooter intentionally introduce a known cant.
Cant: any deviation from a vertical hold position.
Vertical hold: where the scope is held directly above the barrel.
Cant errors have long been known to produce fliers and inaccuracy. Tilting the firearm generates cant error that produces inaccuracy for shooters, especially in competition and long-range hunting. A remedy has been to use an externally mounted bubble level. Some shooters find this acceptable but most find it too difficult to be useful. This test shows the shooter how to identify what the error looks like and how much error is produced.
Each test was designed to explore the ranges commonly used for that type of rifle. Custom targets were designed and printed with computer to assure repeatable and measurable results. The target was designed with a heavy crosshair rotated about the target center by either 10 or 20 degrees. The method of determining the cant was to set the top target edge horizontal with a carpenters level during setup. The rifle scope is then rotated to line up the retical with the printed bold rotated crosshairs on the target. For the longer range shots where the target was too small to see the rotated lines we used the Microlevel to hold a repeatable position. The level was set in its most sensitive range. A gravity inclinometer was used to reset the zero point on the Microlevel®. By holding the Microlevel on the green indicator we are assured that the rifle is canted to the same value each time. Each rifle was fired at three different distances, a short, medium and long range. Each rifle was zeroed at the sighter target with a 5 shot group. Additional 5 shot groups were fired with the rifle canted 10 degrees and 20 degrees. While twenty degrees is a bit extreme, it gives results that clearly illustrate the problem. No one associated with this test believes that you could cant 20 degrees and not know it. 10 degrees might be possible and 5 degrees seems like no cant at all.
Airgun: Anschutz 2002 .177 Cal. Airgun (Modified By Allen Zasandny)
Ammo: Crosman Premier Pellet Wt. 10.5 grains
Velocity: 875 FPS
Scope: 36x Nightforce
Smallbore: Feinwerkbau 2602 .22 LR Caliber
Ammo: RWS Rifle Match
Scope: Leupold (Premier Modified) 8.5x35X
Highpower: Remington 40x Hart Barrel
Ammo: Winchester Varmint extreme Silver ballistic tip 40 grain
Scope: Nightforce 8-32
Airgun 30, 50, 55 Yards
Rimfire: 50, 100 Yards
Highpower: 200, 300, 600 Yards
All grid squares are ½” on test targets
30 Yards 10 Degrees Right Cant; Produces .5” Right .12” Drop Error