What part of the firearm pushes up on the slide stop when the weapon is empty?

The correct answer highlights the role of the follower on the magazine in the operation of a firearm. When a firearm is fired and ammunition is used, the follower is located at the bottom of the magazine and rises as rounds are fed into the chamber. Once the last round is fired and the magazine is emptied, the follower continues to move upward due to the pressure of the spring inside the magazine.

This upward motion of the follower pushes against the slide stop. The slide stop is a component that retains the slide in the rearward position when the magazine is empty, indicating to the shooter that the firearm requires reloading. Therefore, the function of the follower is crucial for the slide stop engagement because it ensures that the shooter is alerted when there are no more bullets ready to be fired. This mechanism is an important safety and operational feature in semi-automatic firearms, allowing the user to remain aware of their ammunition status.