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The simplest of all semiauto pistols is what we commonly call the blowback. Most .22LR, .25 ACP, .32 ACP and .380 ACP semiauto pistols are blowbacks. The usual explanation of this simple design is that the breech is held closed by the recoil spring. This explanation might lead us to believe that little thought need go into the design. The reality is quite different.

At the moment you fire a pistol the powder burns, transforming from a solid to a gas. The combustion gas wants to occupy a much larger space than the powder, perhaps nearly 900 or so times as large. This creates pressure which accelerates the bullet down the barrel. But as Sir Isaac Newton taught us, for every action there is an equal and opposite reaction. In the blowback, the action of pushing against the bullet causes the gas to also push against the cartridge.

Simultaneously the gas pushes against the cartridge case wall and the case head. The gas pressure on the case wall combines with friction to make the case grip the chamber wall, resisting movement of the case. This suggests that the blowback can be thought of as a gas retarded blowback.

Gas pressure on the case head in turn pushes on the breech face, forcing it rearward. As the breech face is pushed rearward, this rearward force must work against two resistances: the recoil spring and the inertia of the slide. As you may have noticed I am neglecting the friction between sliding pistol parts. We are familiar with the recoil spring. It takes a known amount of energy to compress it a known distance.

The inertia of the slide depends on the mass of the material present in the slide. Mass is the amount of material. When gravity pulls a mass toward the ground, we call it weight. Common sense tells us that a heavier slide has more inertia. More inertia means that it takes more force, more "oomph" to open the breech.

What if the blowback has a hammer instead of a striker, as many do? Then the hammer also resists the opening of the breech in two ways. The spring that propels the hammer against the firing pin must be overcome. Additionally, the hammer adds mass that must be pushed aside by the slide.

All of this comes together to make the blowback designer balance the cartrige dimensions, the likely friction between the case and the chamber, the pressure within the cartridge during firing, the strength of the recoil spring, the mass of the slide, the strength of the hammer spring, and the mass of the hammer. And blowbacks seemed so simple.
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