Which of the following is NOT a type of firearm action?

The choice of a pistol as the correct answer is based on the fact that it refers to a type of firearm rather than a specific action mechanism. The term "firearm action" refers to the mechanism by which a firearm loads, fires, and ejects cartridges.

Both lever and pump are classifications of firearm action types. A lever action uses a lever to load and eject cartridges, while a pump action requires the shooter to manually slide the fore-end of the gun back and forth in order to chamber a new round. Similarly, a semi-automatic action allows the firearm to automatically load the next round after firing, using the energy from the fired round to cycle the action.

In contrast, a pistol encompasses a variety of handguns that can operate using different actions (including semi-automatic and revolver actions) and doesn't represent a distinct action style on its own. Therefore, it stands out as not fitting with the other choices that specifically refer to distinct mechanisms of action.