Semi-automatic pistols utilize the pressure generated from the ignition of the gun powder in the cartridge to perform a series of events, which include loading the cartridge, preparing the firearm for discharge, and expelling spent ammunition.
Semi-automatic pistols have a frame where a slide is mounted on a set of rails (which are a part of the frame) and can freely move in a backwards and forward motion on a horizontal plane. The barrel can be attached to the frame, in which case the slide is located to the rear of the barrel while others will mount the barrel within the slide. Regardless of the design, a vertical face, known as the breech face, butts up against the rear end of the barrel. In a locked-breech design, the barrel locks to the slide using lugs that are present in the recesses of the slide design, leveraging the physical interference of a shoulder on the barrel with the rear edge of the ejection port of the slide or other method. The slide houses the firing pin and the extractor which discards used cartridges after they have been fired. Some extractors can be adjusted for pitch and angle of discarded cartridges while others are "fixed" and cannot be adjusted. This is important when purchasing a semi-automatic pistol because if the shells are discarded inappropriately, such as towards your face and body, if the extractor is "fixed", there's little resolution to the problem. An ejection port provides a means in which the extractor may dispense the spent cartridges from the chamber. Semi-automatic pistols may utilize an external hammer, internal hammer, or a spring-loaded striker or firing pin.
Types of Semi-Automatic Pistol Mechanisms
There are three types of semi-automatic pistol mechanisms: Blowback, Recoil, and Gas operated.
Blowback Action Pistols
Blowback semi-automatic pistols utilize a heavy slide and a strong recoil spring to keep the action closed when not in use. When fired, the gasses which build up in the chamber escape towards the rear which pushes the slide backwards, compressing the recoil spring while the ejector dispenses the spent shell. As pressure subsides, the recoil spring moves the slide forward, closing the action while loading a fresh round of ammunition into the chamber for the next firing sequence. Since the barrel is not mechanically locked by the slide, semi-automatic pistols that use the blowback action are usually limited to smaller calibers, such as .22LP, .25 ACP, .32 ACP and .380 Auto.
Recoil Action Pistols
The recoil-operated semi-automatic pistol, the barrel is locked to the slide. When the gun is fired, the pressure from the gasses escape towards the rear of the barrel, causing the slide to be pushed backwards, but unlike the blowback action pistol, the barrel is locked into a horizontal position until the very furthest point of the rear ward slide is reached. When this point is reached, the barrel tips up slightly to accommodate a new round from the magazine to be placed into the chamber. The slide begins to move forward lowering the barrel into a ready position for the next firing sequence. Recoil-operated pistols can withstand higher pressure ammunition and are generally found on larger 9mm, .45, and higher calibers.
Gas-operated actions allow the high-pressure propellant gas to bleed through a small hole in the barrel. The released gas exerts pressure on to a piston or other mechanism driving the slide rearward to unlock the breech and load a new round into the chamber.
Semi-automatic pistols are most famous for their delivery of fresh ammunition during the firing process. A component known as the magazine is a detachable metal container which holds the ammunition in the frame, stacked on top one another just below the barrel. A tension spring applies pressure to the ammunition in an upwards direction towards the barrel ready to slide the next available cartridge into the chamber when the slide is pulled or the gun is fired. Magazines are easily ejected and replaced in one sweeping action, making the semi-automatic a popular self-defense weapon.
There are three types of trigger mechanisms available for the semi-automatic pistol: Single-Action, Double-Action, and Double-Action Only. Semi-automatic pistols come in a variety of shooting styles, which include: An external hammer, and internal hammer, a spring-powered striker, or a spring-powered firing pin. Unlike a revolver, the firing process for a semi-automatic pistol would continue to occur as long as there is ammunition in the magazine and the trigger is pulled back in a permanent hold position. For this reason, semi-automatic pistols as a mechanism which disconnects and cancels the firing sequence until the trigger "reset" to a pre-determined position. This prevents the semi-automatic pistol from being a completely automatic, machine gun style pistol.
Single-Action Semi-Automatic Pistol
The single-action semi-automatic pistol works on the same principle as the single-action revolver discussed earlier. The single-action pistol requires the hammer be cocked before firing with no benefit of the automatic features you would expect.
Double-Action Automatic Pistol
The double-action automatic pistol works on the same principle as the double-action revolver discussed earlier. Most double-action semi-automatic pistols will operate as both a single-action pistol and as a double-action pistol. The double-action trigger pull is longer and requires more pressure to fire the gun. This is because during the pull, the trigger cocking the hammer from a closed position to a battery ready position and then releasing the hammer when the trigger is completely pulled, Provided a round of ammunition is present in the chamber, the slide mechanism will "ready" the hammer automatically, allowing for the pistol to be fired when the trigger is pulled again. This second pull will require much less pressure as the hammer is already in position for firing.
Double-Action Only Pistols
Double-action only pistols are exactly what the name implies--they are pistols that will only fire in double-action mode, which work on the same principle as the double-action pistol described earlier. This means each shot requires the long pull and the additional pressure on the trigger to commence firing a round. These pistols are considered less-likely to have an unintended discharge do to the extra effort required to fire the weapon. While many feel this type of gun is safer, others feel it slows the reaction time down considerably, especially during a life threatening encounter.
Semi-Automatic Pistol Safety Mechanisms
Safety feature take on a variety of options depending on the manufacturer. For simplicity sake, we will discuss the major safety features that are most common for the semi-automatic pistol.
Pivoting thumb safety mechanisms are the most common safety. These safety switches are located, typically on the left side of the gun however modern manufacturers are now locating them on both sides of the gun for both left and right handed shooters. In general, if the switch is in the up position, it is typically engaged, preventing the firearm form discharging whereas in the down position the safety is off, allowing the firearm to be discharged. Unfortunately, this is the general rule, there are exceptions; be sure to check your owner's manual for a complete explanation of the safety switch should your gun have one. Other types of safety mechanisms can include those that work to block the sear or the slide while others prevent the hammer from contacting the firing pin. The hammer drop safety, a special form of safety switch, allows gun owners to safely decock their gun when it contains a live round in the chamber. When this is engaged, the hammer falls into the lowered position. Other safety mechanisms include the trigger safety, the grip safety, and the magazine disconnect which prevents firing a round if the magazine is removed.
Order of Operations
When the pistol is first loaded there are no rounds in the chamber. To load a round the slide is quickly pulled back allowing the ammunition in the spring loaded magazine to load into the chamber in the firing position. This action also locks the trigger or striker bar into the ready position. When the trigger is pulled, the firing pin is struck by the hammer (or the striker pin is released) which strikes the primer cap on the cartridge. The primer ignites the gunpowder which then causes the bullet to escape from the barrel of the gun at a high velocity. After the bullet has left the barrel, the released gasses from the gunpowder forces the slide rear ward, which cocks the hammer for the next round. During this process, the ejector pin grabs the spent shell and ejects it via the ejection port. The magazine replaces the spent cartridge with a fresh round as the slide is returning to its original resting state. The trigger is reset to the firing position, allowing the shooter to continue firing rounds.