Typically, a sniper rifle is used at long ranges with the assistance of the rifle scope, but, generally, use of the scope occludes all peripheral vision as well as slowing the character using the weapon down. While not looking through the sights, there is typically a lack of heads-up display (HUD) crosshairs intended to prevent meaningful use of the sniper rifle in close combat.
Some people are good at estimating where the center of the screen is (the typical location of a HUD crosshair, where the shots will go), while others may use something such as software that overlays a crosshair (typically against the license agreement of multiplayer games, and may be detected as a hack by anti-cheat software) or a dry-erase or overhead transparency marker on the screen, placing a dot where the crosshair would typically be. No matter what method is used, the no-scoper then uses the sniper rifle without using its attached scope, making it more useful for close-combat.
Using such a weapon in close combat is typically against the intentions of the designer, who intend for the weapon to be used at range while aiming through the scope. Different techniques are often used to discourage no-scoping, such as the lack of a HUD crosshair, wild inaccuracy while unscoped, and in some cases, such as in Team Fortress 2, the sniper rifle only becomes powerful after looking through the scope, and unscoped shots do minimal damage.