The Barrett is a Sniper Rifle weapon, that very often deals enough damage to instantly kill a player, though it only has one shot before it must be reloaded. The reload time itself takes a very long time, and leaves you very vulnerable. The weapon is best used while laying down because it makes you harder to hit. While crouching or laying down, if you hold the fire button, the camera will follow your bullet until either you let go, or the bullet disappears. The crosshair for the Barrett will shrink or enlarge depending on how fast your character is going.
(Tip for using the Barrett: try to carry anything else you can, whether it is a weapon someone else dropped, a secondary, or a throwable, so that you are not completely vulnerable)
(Tip for combating the Barrett, try to move fast and force them to waste their one shot, so that they are momentarily vulnerable)