AR-15 Handbook. Erik Lawrence

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      Figure 3-1 Slant Range

      CHANGING LIGHT CONDITIONS

      Bright, sunny day vs. cloudy or overcast day, sun in different positions (may affect the zero of the weapon because of the change in the shooter’s perception of the target. This is less of a factor with a red dot sight than with iron sights.) On bright days, shooters tend to hit low, and on cloudy days, they tend to hit high. An easy-to-remember phrase is “lights up, sights up; lights down, sights down.”

      CANT OF THE WEAPON

      (close range vs. long range) – If you cant the weapon while firing (around obstacles or under a vehicle, etc.), you will move the strike of the bullet in the direction of the cant and low (i.e., cant the weapon to the right – the round strikes low and right). This is because when you cant the weapon, the windage becomes the elevation, and the elevation the windage. At close range, it will only be the difference between line of sight and line of bore, but at distance, you will have to adjust for bullet drop as well. At 100m with your rifle canted 90 degrees to the left, you can expect the strike of the round to be 4”-6” left and 4”-6” low. At 100m canted left, aim at the right shoulder. Try not canting your weapon unless you have to, but if necessary, hold in the opposite direction and high.

      FREE-FLOATED BARRELS

      These are guns, usually sniper systems, or Special Operations Peculiar Modifications on M4A1 carbines, where the barrel does not touch anything but the point where it is screwed into the receiver. This will give it consistent barrel harmonics and tremendous accuracy. The barrel flexes when the rifle fires, and if the rifle is not free floated, it will not flex consistently. If you rest your handguards on a wall or barricade, that will mitigate much of the problem. If you rest the actual barrel of your AR-15 on a wall, fence, or object of some type, the strike of the round will be high about 12”-16” at 100m. If you are leaning down on it, you can double that at 100m. Keep your barrel off of objects if at all possible. It will throw your rounds off dramatically.

      NOTE

      Try not to cant your weapon unless you have to, but if necessary, hold in the opposite direction and high (i.e., at 200m leaning 90 degrees to the left, hold on the right shoulder of an E-type silhouette target or enemy.)