In firearms terminology, an action is the functional mechanism of a breechloading firearm that handles (loads, locks, fires, extracts, and ejects) the ammunition cartridges, or the method by which that mechanism works. Actions are technically not present on muzzleloaders, as all those are single-shot firearms with a closed off breech with the powder and projectile manually loaded from the muzzle. Instead, the muzzleloader ignition mechanism is referred to as the lock (e.g. matchlock, wheellock, flintlock, and caplock).

Actions can be categorized in several ways, including single action versus double action, break action versus lever-action, pump-action, bolt-action, among many other types. The term action can also include short, long, and magnum if it is in reference to the length of the rifle's receiver and the length of the bolt. The short action rifle usually can accommodate a cartridge length of 2.8 in (71 mm) or smaller. The long action rifle can accommodate a cartridge of 3.34 in (85 mm), and the magnum action rifle can accommodate cartridges of 3.6 in (91 mm).

Single-shot actions operate only to ignite a cartridge that is separately set up ("in battery") for firing, and are incapable of moving the cartridge, itself. As the name implies, all single-shot firearms (unless they are multi-barreled) can only hold one round of ammunition and need to be manually reloaded after every firing. Historically, these are the earliest cartridge firearm actions invented.

The dropping block are actions wherein the breechblock lowers or "drops" into the receiver to open the breech, usually actuated by an underlever. There are two principal types of dropping block: the tilting block and the falling block.

In a tilting block or pivoting block action, the breechblock is hinged on a pin mounted at the rear (in contrast with tilting bolt, which is not hinged). When the lever is operated, the block tilts down and forward, exposing the chamber. The best-known pivoting block designs are the Peabody, the Peabody–Martini, and Ballard actions.

The original Peabody rifles, manufactured by the Providence Tool Company, used a manually cocked side-hammer. Swiss gunsmith Friedrich Martini developed a pivoting block action by modifying the Peabody, which incorporated a hammerless striker that was cocked by the operating lever with the same single efficient motion that also pivoted the block. The 1871 Martini–Henry which replaced the "trapdoor" Snider–Enfield was the standard British Army rifle of the later Victorian era, and the Martini is also a popular action for civilian rifles.

Charles H. Ballard's self-cocking tilting-block action was produced by the Marlin Firearms Company from 1875 and earned a superlative reputation among long-range "Creedmoor" target shooters. Surviving Marlin Ballards are today highly prized by collectors, especially those mounted in the elaborate Swiss-style Schützen stocks of the day.

In a falling block or sliding block action, a solid metal breechblock "slides" vertically in grooves cut into the breech of the firearm and actuated by a lever. Examples of firearms using the falling-block action are the Sharps rifle and Ruger No. 1.