Among the wide variety of firearms, and particularly handguns, semi-automatic pistols are surely not, by far, the most technically understood. Not to speak of the fairly frequent confusion between pistols and revolvers.

In this section, will be laid down some technical basis to clarify what is a semi-automatic pistol and how it functions. And first of all, by opposition, you will find hereafter a first picture representing an old Belgian "bulldog" revolver chambering an obsolete .38 black powder cartridge. This kind of small pocket handgun, with a folding trigger, was very frequent at the end of the 19th century when semi-automatic pistols were yet to be produced. As you can see a revolver bears a revolving cylinder drilled out of chambers to accomodate cartridges. When you face a revolver you can immediately see if it is or not loaded as the bullets' face is exposed. It is a quite interesting "advantage" that does not occur when facing a pistol. Even if facing any one of them, anyway, does not inspire confidence.

Semi-automatic pistols do not possess a revolving cylinder. Generally, the ammunitions are contained in a movable magazine which, in most cases, is slid in the pistol's butt. It is therefore rather difficult to know if a pistol is loaded with a magazine, and moreover, if this latter really includes cartridges. Not to speak of the uncertainty about the real presence of a cartridge in the barrel's chamber, as a pistol can hold a full magazine in the butt without a cartridge effectively chambered. Many things to be understood for a layman, and many things that make the semi-automatic pistol more dangerous.

As one can see on the pistol hereover pictured (Colt model 1908), the barrel is most often not exposed and surrounded by a moving part : the slide. To chamber a cartridge, it is mandatory to first draw the slide back to its rearmost position, by gripping it through its serrations, and let it slam forward under the recoil spring pressure. When moving back to its initial position, the breech's face (being an integral part of the slide) strips a cartridge out the magazine and pushes it in the barrel's chamber. The pistol is then ready for firing. At each shoot, the rearward movement of the slide is induced by the gazes pressure against the breech face through the cartridge case. The cartridge rechambering is automatically performed during the firing cycle. Why then the "semi-automatic" term? Technically, an "automatic" firing occurs when cartridges are loaded in a continuous stream as long as the trigger is kept under the finger's pressure, like a machinegun for instance. A semi-automatic firearm operates shoot by shoot, and the shooter has to release his finger between each one. Generally, "automatic" pistols are in fact "semi-automatic", but this latter term is seemingly quite few in use (probably too long) when speaking about this firearms category, except by specialists who want to be clearly understood by an exacting audience.

When observing the semi-automatic pistols functioning, one can point out many mechanical principles which are all the more sophisticated that the cartridge becomes powerful. There are some differences in the general strength and mechanical disposition if a pistol has to overcome the power of a .25 ACP cartridge or cope with the potent .45 ACP. As soon as we approach the .38 (9 mm) caliber, it is hazardous to oppose the slide weight only to the gazes pressure. This is approximately at this cross point that well-known firearm designers proposed technical solutions that characterize their genius. Anyone initiated  in the field may recognise at once a Browning or Walther firearm type, simply by the presence of technical options that were created  by these inventors. For instance, the Beretta 92 pistol encloses a pure Walther P38 locking device even if conserving many of the external traditional Beretta pistol's features.

Among the most original systems was, no doubt, the toggle lock initiated by Hugo Borchardt and greatly improved by Georg Luger. The Luger pistol, in its achieved form, date back to 1898 and remained in service in Germany until the end of   World War II. Swiss kept it as a regulation sidearm for many more years.

When a cartridge releases a pressure nearing or passing one Ton/cm2, it must be understood that the slide can be considered as a second missile oriented toward the shooter's face. To deal with that somewhat dangerous issue there is only one solution : maintain the slide and barrel jointly locked for a portion of time that allows the bullet to leave the barrel and the pressure consequently drop to zero.

In the Luger pistol, the breech's locking is warranted by a toggle mechanism. The barrel is screwed in a long U shaped barrel extension. Inside this extension can travel a mobile breech block pinned to the forward link of the toggle. The rear link being fixed to the extension through a large pin. So, only the breech block can move horizontally between the extension branches inside the limits of the toggle folding.

In closing position, that is to say with the two toggle links flat, the "barrel-extension-breech-toggle" acts as one closed bound unit. This state of thing continues until the center of the mid-axis remains under the line that joins the centers of the external axes (the forward one linked to the breech block and the rearward one linked to the barrel extension, as depicted hereafter).

Upon firing, under the burning gazes thrust, the whole bound unit moves quickly rearward. During this move the bullet has enough time to leave the barrel. The pressure then drops instantly to zero. Quite evidently the moving parts keep an important momentum. After a 6 mm move, the toggle knobs are reaching a slope corresponding to the frame upper profile (green arrow).

Progressively, as the knobs climb the slope, the central axis center goes up to finally reach a position (over the line of the two other axes) where the toggle is mechanically induced to fold, and consecutively draw the breechblock backward through its front link connection. The extraction and ejection of the case take place at this moment.

When the toggle has terminated its maximum folding, the recoil spring acts through a specific link to drive the toggle forward and close lock the pistol. The breech face strips a fresh round out of the magazine and chambers it. The firing cycle is then achieved.

This whole complicated mechanism has only one vocation : overcome the heavy pressure released by the cartridge. This system here depicted is typical of the Luger pistol, the .45 Colt Browning pistol encloses another system, as does the Walther P38 or the 1907 Roth Steyr, but the goal remains the same : to lock the barrel and the slide for a short while in order for the bullet to leave the barrel and the pressure to drop to zero.

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