What keeps the slide and barrel of the M9 open after the last round has been fired?

The correct response highlights the role of the magazine follower in the functioning of the M9 pistol. Once the last round in the magazine is fired, the magazine follower rises to the top of the magazine due to the absence of additional cartridges. This upward movement of the follower activates the slide stop, causing the slide to lock back in the open position. This feature is essential as it indicates to the shooter that the magazine is empty, allowing for a quick and efficient reload.

The extractor, while crucial for removing spent cartridges from the chamber, does not contribute to keeping the slide open after the last round is fired. The slide release is actually a mechanism that allows the shooter to manually release the slide, but it does not automatically keep it open after the last shot. The safety lever serves as a safety feature to prevent accidental discharge but does not influence the position of the slide post-operation. Hence, the function of the magazine follower is key to understanding the operation of the M9 when the last round is fired.