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How does recoil operation differ from blowback operation in machine guns?

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Semiautomatic pistol — small arm Glock .40-calibre semiautomatic pistol.

In machine gun technology, recoil and blowback are distinct methods for harnessing the energy of a fired cartridge to automate the weapon's cycle of operation.1

Blowback Operation

In blowback operation, the empty cartridge case is propelled backward by the force of the explosion. This rearward movement directly pushes the bolt (or breechblock) backward, which compresses a spring. The energy stored in this spring then returns the bolt to the firing position. The primary challenge in this design is controlling the bolt's motion to ensure that loading, firing, and ejection occur in the correct sequence.1

Recoil Operation

In recoil operation, the bolt is locked to the barrel at the moment of firing. When the round is discharged, both the bolt and the barrel recoil together. Once the pressure in the barrel drops to a safe level, the locking mechanism releases the bolt. The barrel is then returned forward by its own spring, while the bolt remains held to the rear until a new round is positioned in the open breech.1