sniper
A sniper is marksman trained to spot and pick off enemy soldiers or other important targets from a concealed position. Unconventionally, snipers may be fielded in secret operations where a target is too risky to take out up close, or when the anonymity of the shooter and those who sent them is important.
Most commonly, however, snipers are used in open warfare, in which their distance and concealment from the enemy allows them to conduct reconnaissance and provide supporting fire for friendly troops, a tactic known as overwatch. Modern conventional snipers almost always work in pairs, with one shooting and the other spotting for them.
The term originates from hunting birds known as snipes, which are notorious for moving erratically. Since it took an exceptionally skilled marksmen to shoot them, the term stuck to sharpshooters in general.
A sniper's weapon of choice is, of course, a sniper rifle. Do not tag sniper unless such a weapon is being wielded (in other words, it should not be on a weapon rack or sitting on the ground). The tag refers to the wielder, not the weapon itself.