Actions: Blowback Versus Locked Breech
Self-loading automatic pistols can be divided into "blowback" and "locked breech" categories according to their principle of operation. The blowback operating principle is suitable for smaller, low-powered calibres, such as 7.65mm Browning (also known as .32 ACP), and 9mm Browning Short (also known as .380 ACP) as the resistance of the recoil spring and mass of the slide are sufficient to retard the opening of the breech until the projectile has left the barrel, and breech pressure has dropped to a safe level. For more powerful calibres such as the 9mm Parabellum (9mmP) and .45 ACP, some form of locked breech is needed to retard breech opening, as an unlocked blowback pistol in these calibres requires a very heavy slide and stiff spring, making them bulky, heavy and difficult to operate. A somewhat commercially successful blowback pistol design in the more powerful calibres was produced; the Spanish Astra 400 in 9mm Largo and the similar Astra 600 in 9mm Parabellum. U.S. manufacturer Hi-Point also produces a line of blowback operated pistols in several calibers including 9mmP and .45 ACP. Virtually all other service-caliber pistols are locked breech designs.
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