How does a gas-operated machine gun use propellant gases to reload?

In a gas-operated machine gun, the energy required for automatic cycling is derived from the high-pressure propellant gases generated when a cartridge is fired.1

The process functions as follows:

• Gas Diversion: A small port or opening is located in the side of the barrel, positioned between the breech and the muzzle.1
• Piston Activation: Once the bullet passes this port, a portion of the high-pressure gas is tapped off and directed into a cylinder. This gas exerts pressure against a piston or similar device, converting the gas pressure into a mechanical thrust.12
• Cycling the Mechanism: This thrust drives the piston to the rear, which unlocks the breechblock and pushes the bolt backward against a main spring.12
• Reloading: As the bolt moves, it extracts and ejects the spent cartridge case. The energy stored in the compressed spring then drives the bolt forward, picking up a fresh round from the ammunition supply and chambering it for the next shot.12

This system allows the weapon to perform the necessary functions of loading, firing, and ejection without requiring an external power source.1