How do semiautomatic pistols work?

The colourful profi le of the semi-automatic weapon continues to shape public opinion, but there is more to its substance than style alone.

The semi-automatic pistol is a functionally different animal to the romanticised revolver of the Wild West. The motivation for semi – and full for that matter – automatics derive from energy generated by the fi ring process to self-load and prime a new round. This comes in a variety of fl avours, including recoil, blowback and gas. Recoil is the gun’s kick-back, balancing the bullet’s forward momentum – or as Newton says, with every action must come an equal and opposite reaction. Here, the opposing recoil force drives the gun backwards, initiating momentum in the ‘slide’ and barrel that are mechanically engaged. Separation of the two typically allows the breech to open as the slide carries on, self-loading and cocking the gun in the process. 

With blowback the barrel and slide are not wed. The barrel is typically fi xed to the frame with the shunting force of the exploding cartridge operating against the breech face itself and forcing the slide to the rear. The infamous AK-47 is a further example of a system that siphons gas drawn from the fi red cartridge explosion to cycle the self-loading process.  Despite these distinctions, the term automatic is often clouded with reference to loading and fi ring. Though its function is distinct from its ancestors, the triggering mechanism of semi-automatics such as the US Army’s M1911 mean they can only discharge one round for every reciprocal pull of the trigger. This differentiates them from full automatics which utilise a trigger mechanism that actuates a continuous self-loading/fi ring cycle until a gun’s clip is spent or trigger released.

Due to the unwieldy nature of full automatic pistols, semiautomatic variants are now common throughout the military, police and criminal underworld..

1. Cock 

The weapon is first primed by manually racking the slide, which cocks the hammer and chambers the round.

2. Squeeze 

The hammer is held by a small notch or ‘sear’. Upon pulling the trigger the sear moves and the spring-loaded hammer slips free, striking the firing pin which in turn hits the primer.

3. …fire

 The primer explodes the gunpowder, sheaving the bullet from its case. Expanding gases force the bullet down the barrel past helical grooves that impart spin to improve accuracy in flight. 

4. Shots away!

 Combustion gases provide muzzle velocity upwards of 250m/s; in turn the slide recoil is locked to the barrel by ‘lugs’. As the bullet exits, bore pressure falls.

5. On the slide 

At this point the ‘link’ pivots the barrel out of lock and the lugs disengage. The slide continues to retreat under conserved momentum, compressing the recoil spring.

6. Up and out

 The breech opens, the extractor and ejector take turns to draw and kick out the spent chambered cartridge. The slide continues passing over and recocking the hammer.

Semi vs fully automatic 

While both loading mechanisms are automated, the advantage of going full automatic means there is no trigger disconnect and no mechanical delay in the cycling of fire representative of semi-automatic weapons. Therefore, while they are great in a tight spot and satisfy a penchant for wanton carnage, such continuous fire – allied to a typically low weight and no shoulder stock – makes them tough to control, and a tendency to kick-up during firing makes them prone to vertical spray. ‘Cook-off’ is also a factor in full automatics, where a round may dispense prematurely from the over-heated chamber. 

Full automatics often benefit from an open bolt policy, where the slide is held back at the end of the cycle to allow cooling air to filter the barrel. Another issue is slam fire. This occurs when the slide is released and the force of it closing is powerful enough to detonate the primer. They are also subject to jamming, where the cartridge can stick while entering, or ejecting from the chamber.