Firearms employing gas pressure loading mechanisms are disclosed. An example firearm includes a single force receiving component containing a gas cylinder, and a barrel in communication with a cartridge chamber received in the force receiving component. The cartridge chamber is in communication with a gas withdrawal-opening and is sized to fire cartridges having a caliber of at least 15 mm. The firearm also includes a bore in communication with the gas withdrawal opening and the gas cylinder. Further, the firearm includes a locking bolt having a locked position and an unlocked position. The locking bolt engages the single force receiving component when the locking bolt is in the locked position.