Before the action or slide begins to move rearward in a gas-delayed blowback firearm, what must occur?

In a gas-delayed blowback firearm, before the action or slide begins to move rearward, it is essential for the case to push against the bolt or slide. This interaction occurs as a result of the expanding gas created from the burning gunpowder in the cartridge. When a round is fired, the pressure from the gas generated in the chamber forces the case against the bolt. This initial movement is critical because it ensures that the pressure from the gas is sufficiently harnessed to delay the opening of the action, allowing for proper extraction, ejection, and cycling of the next round.

This mechanism is fundamental to the operation of gas-delayed systems, as it momentarily holds the bolt in place until the pressure drops to levels allowing safe extraction of the spent cartridge. The correct functioning of this process is vital for reliable firearm operation and contributes to the overall efficiency of semi-automatic and automatic firearms that utilize this design.

The other options, while related to firearm operations, do not specifically address the immediate necessary action that must take place before the action begins moving rearward in this type of firearm. The action of the case pushing against the bolt is what initiates the cycling process specifically for gas-delayed blowback systems.