Most people do not understand how semi-automatic firearms work. Here is a basic lesson to explain the mechanics that allows them to automatically eject an empty cartridge and reload a new bullet into the chamber.
The semi-automatic firearm is so standard now that many accept it as the way firearms have always worked. Truth is, getting a workable semi-automatic was as groundbreaking as the repeating firearm or center-fire primer. Like so many innovations, the semi-automatic firearm is older than most think, but it took a while to work out the wrinkles enough to get widespread attention. Ferdinand Ritter von Mannlicher, an Austrian engineer and small arms designer, is credited with the first semi-automatic firearm—a rifle—in 1885. Mannlicher also designed a number of bolt-action rifles and semi-automatic pistols, as well as the en-bloc clip. The first self-loaders were blowback operated; later on came recoil and gas-operated guns. Let’s examine how each of these systems work.
Blowback operation is the simplest of the semi-automatic operations. It relies on a combination of slide or bolt mass and spring tension to keep the slide or bolt in place until the bullet leaves the muzzle. The force generated by the combustion of the powder eventually overcomes the inertia of the slide or bolt, forcing it rearward using the fired case as a sealer. A recoil spring returns the slide or bolt into battery while chambering a new cartridge from the magazine. Because there is no mechanism to lock the bolt in battery, straight blowback-operated guns are relegated to low-power cartridges, primarily rimfires and pistol cartridges up to the .380 ACP.
A variation called delayed blowback is used in guns chambered for more powerful cartridges. Delayed blowback firearms use either a lever or roller to restrain the bolt movement. In a lever-blowback firearm, a lever on the bolt carrier moves toward the rear at a rate faster than the bolt. That lever acts against a surface via a cam or incline that increases the leverage to restrain the bolt, thus slowing its velocity. When the breech pressure drops enough due to the bullet leaving the muzzle, the bolt overcomes the restraints and moves rearward to cycle the action, returning to battery under spring tension. The most familiar to utilize this system is the 5.56 NATO FAMAS (Fusil d’Assaut de la Manufacture d’Armes de Saint-Étienne).