What is fundamental about the operation of the M2 Browning machine gun?

The operation of the M2 Browning machine gun is fundamentally characterized by its use of a rotating bolt for locking. This design ensures that the bolt securely locks into place when the weapon is fired, which helps to manage the high pressures generated from the combustion of the gunpowder in the cartridge. The rotating bolt mechanism enhances the reliability and durability of the gun, allowing it to perform effectively even under harsh conditions and with a high volume of fire.

In contrast, the other choices describe characteristics that do not accurately represent the M2 Browning. It does not rely on an electric firing mechanism; instead, it is fired using a mechanical trigger system. Additionally, it doesn't operate with a direct blowback method but rather uses a recoil-operated mechanism, which is distinct from blowback designs. Lastly, the M2 is not a semi-automatic weapon; it is designed to function as a fully automatic machine gun, capable of sustained fire. These characteristics contribute to the reliability and lethality of the M2 Browning machine gun on the battlefield.