Operating Systems

Previous Section - Inertia (Blowback)

Delayed Inertia (Delayed Blowback)

In delayed inertia operation the bolt again is not positively locked and the energy to cycle the system comes from recoil forces driving the bolt rearward. The difference is that initial opening of the breech is delayed, typically through mechanical disadvantage.

This delay can be accomplished in a variety of ways. For example the WWI-era Schwarzlose machine gun has links between the bolt and receiver which require kinetic energy to rotate. Due to their arrangement, as the bolt first moves these links start to turn, absorbing rotational energy. As the bolt moves back further, the leverage changes and the energy in the links is transfered to the bolt.

Another method of delaying blowback is to require small masses, usually in the bolt, to move at near right angles relative to the bolt just after firing. While these masses appear to lock the bolt to the receiver they actually act as levers to effectively multiply their mass and the bolt's against internal recoil forces.

The most successful modern examples of delayed inertia or delayed blowback arms are the Heckler & Koch G3 rifles and MP5 submachine guns and their derivatives. In the 1950s HK licensed this design by former Mauser engineer Vorgrimmler who was then working at the Spanish CETME laboratory. Vorgrimmler reportedly derived the CETME rifle from his earlier work on the WWII-era Mauser model 45 assault rifle.

HK describes their operation as roller-locked, delayed-blowback. In these designs when the bolt is closed, rollers carried in the bolt are wedged into receiver recesses. On firing, the rollers must be forced out of the recesses at great mechanical disadvantage, delaying opening of the bolt, even with full power 7.62mm NATO (.308 Winchester) rifle cartridges used in the G3. The G3 and MP5 fire from a closed bolt and are manufactured to very high standards resulting in excellent accuracy, especially as mass produced military arms. Despite utilitarian stamped and spot-welded sheet-metal construction and painted finishes, their simple and smooth operation make them highly reliable and comfortable to fire.

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