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"Lock up" refers to the fact that the slide does not move backwards immediately when a cartridge is fired. On a semi-auto, the breech face is machined into the slide, and it is important that the breech remains in battery (forward position) for a split-second until the pressure level in the chamber drops to a safe level. The "lock up" of a pistol is critical to prevent the shooter from receiving an equal and opposite reaction square in the mug.
Ignoring some of the more unusual designs for the moment (such as the HK P7 series), semi-auto pistols chambered in 9mm Parabellum (9x19, 9mm Luger) and up require a locked-breech action, because of the higher pressures generated by the cartridge. John Browning made this system practical, and the vast majority of pistols of 9mm Para and higher calibers use some variation of his designs.
Browning System: The barrel is a separate component from the frame, and it can be easily removed. Basically, one to three locking lugs are machined into the top of the barrel. When the pistol is fully asssembled and in battery (slide in the forward, ready to fire position), these lugs match up to corresponding grooves machined in the slide. When the pistol is fired, both the barrel and slide move backwards, "locked up," for a few millimeters due to the gas pressure on the breech face. In the original version for the Colt 1905/1911, the barrel has a swinging link. As the barrel reaches this hinge point, it will drop down, releasing the lugs from the grooves. It is retained in place by the slide stop while the slide continues to move backwards. The slide reaches its maximum rearward travel, and it is returned forward by the stored energy of the recoil spring. The momentum of the slide moves the barrel forward, restoring the lock-up.
There have been several refinements on this design. The so-called "Improved Browning" system dispenses with the swinging link and uses a shank on the bottom of the barrel. This shank has a machined cut that matches up with the slide stop and performs the same function as the link. This was introduced on the Hi-Power. The Browning-Petter system dispenses with the locking lugs. Instead, the chamber is surrounded by a massive block, which is shaped to fit the ejection port so as to create a complete lock-up. This block on the barrel matches up to corresponding cuts on the slide.
The locked-breech systems can handle higher pressures and velocities becasue the locked barrel/slide combination has a much greater resistance to the gas pressure reaction against the breech face. These "locked" systems also serve as a sort of timing mechanism. The mechanical steps involved happen too quickly for the naked eye, yet they do delay the cycle long enough for the bullet to leave the barrel and the pressure levels to drop. Otherwise, hot gasses could vent from the chamber into the face of the shooter.
Blowback System: This is the system used by the Makarov and other pistols of 9x18 Makarov and smaller calibers. In this system, the barrel is more or less permanently mounted on the frame. (Barrels can be changed, but it is not a simple "field strip" operation.) In a blowback, there are no locking lugs or similar features. Properly speaking, these pistols do "lock up," but you will almost never hear that term used in conjunction with a blowback action. That is, the slide does not begin to move backwards immediately.
In a blowback, the weight of the slide, the strength of the recoil spring, and the length of the barrel must correspond to the load. As the cartridge is fired, the case makes a gas seal. The pressure pushes the case backwards against the breech face as the bullet moves down the barrel. The pistol is designed in such a way that the weight of the slide and the strength of the recoil spring prevent the slide from moving until the bullet has exited the muzzle. (The slides of blowback pistols are usually quite massive for their size, and the weight is usually distributed to the front.) If one of these factors gets out of balance, a blowback can easily pose a danger to the shooter. Also, loads that do not match the design correctly may function poorly, even if perfectly safe. It is important that your Makarov has a good, strong recoil spring, since it needs to exert quite a bit of tension forward to maintain the "lock up."
I hope this helped. If you watch someone field strip almost any 9mm Para pistol, this explanation should make sense.
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WARNING: CZs MAY BE HABIT-FORMING (Consult a doctor if nursing or pregnant).
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