After the rifle itself, the second major component of the sniper-rifle weapon system is the sniper scope. A sniper scope is basically a specialized telescope containing components that lay a targeting reticule (crosshairs) over the amplified image.
When sighting a target through a scope, snipers are comparing point of aim to point of impact. Simply put, when firing a bullet from over 600 yards, where you are aiming is not going to be where the bullet lands. All sorts of variables work on that bullet during its long flight to the target. Ideally, snipers want point of aim and point of impact to be the same. They line up these points with fine adjustments to the scope once range, heat and windage have been factored into the shot.
Photo courtesy Department of Defense Defense Visual Information Center
The Unertl sniper scopes used by the U.S. Marine Corps house the optics in steel tubes that are mounted to a bracket on the top of the rifle. They weigh 2 lbs, 3 ounces (~1 kg) and are 10 inches (~25 cm) long. They are fixed, 10-power scopes with a 32mm objective lens. This means that they are capable of magnifying an image to 10 times its size. The sniper uses the wire reticule with mil dots to range and sight the target. The mil dots surround the target center and allow the sniper to estimate the distance between objects and make adjustments for wind or moving targets.
These scopes sport ballistic drop compensators (BDC). The BDC looks like a small, round dial and helps the sniper adjust the scope to compensate for battlefield variables as well as the natural behavior of these rounds in flight. With the BDC, snipers can make fine changes to the scope without touching the range settings. A sniper can adjust for any range up to 1,000 yards, as well as make adjustments up, down, left or right.