Rifle slings have been used for marksmanship training in the U.S. military for many years. A soldier or marine typically use a three-point sling as a transporting device, enabling him to shoulder and carry the weapon while keeping the hands free. The sling is also used to stabilise the rifle while in any given shooting position. The three-point sling makes this stabilisation process more effective than with regular two-point slings, because it creates a loop for arm and shoulder placement that helps secure the rifle into a solid shooting position. Using a three-point sling is easy to master once you've had the proper instruction.
Loosen the sling to give moderate slack around the loop created by the three-point sling. Unfasten the buckle on the loop and pull the strap out slightly, then refasten the buckle.
Hold the rifle in your preferred shooting position. Place your free arm through the loop created by the three-point sling. Bring your forearm back through the loop and grab the rifle hand-guard. The loop should be cinched around your shoulder and forearm. Your free arm is the arm that doesn't control the trigger.
Tighten the sling down around your forearm and shoulder by loosening the buckle and pulling the strap. Replace the buckle when your sling is tight and restricts your arm movement slightly.
Wedge the butt-stock of the rifle in between your cheek and shoulder, while gripping the hand-guard with your free arm. You will notice that this position is extremely tight and allows for barely any movement of the rifle or your free arm.
Purchase a three-point sling that corresponds to your model of rifle. Many rifles do not have a third-sling swivel, or attachment point. If you have this type of rifle, you will need to have your rifle modified. You may have to attach a sling swivel or tie one point of the sling to your rifle.
Always keep the barrel of the weapon in a safe direction, and the weapon on safe while adjusting your sling. Never point a weapon at anything that you do not intend to shoot or kill.