Abstract: A gas-operated firearm is disclosed in which the gas port is located in the forward part of the chamber, in the part of the barrel bore wall which surrounds the portion of the bullet projecting from the case of a chambered cartridge. In the disclosed firearm the bolt remains locked in its breech-closing position until the gas pressure resulting from the detonation of a cartridge drops to practically zero (gauge). The bolt, which rides on guide rails in the receiver, is provided with a pivotable locking member constructed and arranged to engage locking recesses in the guide rails when it is pivoted into one of its extreme positions. The locking member can be withdrawn from engagement with the locking recesses in the guide rails, and the bolt impelled rearwardly, when a lug projecting from the locking member is struck by a cam which is pivotably mounted in the receiver.