Rail system (firearms)

Rail systems are small strips of metal put on any given surface on a gun to allow attachment of some sort of accessory. On them, accessories such as laser sights, scopes and R.I.S systems can be attached. They make the mounting and dismounting of these objects significantly easier. Rail systems usually are based on the handguard of a weapon or the Upper receiver. The two types of rail systems for firearms are the Picatinny rail and the Weaver rail.

...
Wikipedia