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Difference Between an HK & CETME Bolt

The Heckler and Koch (HK) G3 rifle was adopted by West Germany in 1959 and is based on the Spanish-made CETME Model A rifle. CETME is an acronym for the Spanish name of the second weapon's developer, Center for Technical Studies of Special Materials. The CETME and the HK G3 rifles operate on the same roller delayed blowback system, which is an alternative to direct blowback.

"Blowback" refers to the operation of a rifle bolt as the weapon is fired. Tremendous pressure propels the bullet forward, and the bolt seals the rear of the barrel, preventing gasses from causing damage to the gun's chamber, where the bullet feeds into the barrel. The bolt can be held forward by a spring and heavy bolt, which keeps the chamber sealed, or by the delayed blowback used in the HK G3, in which the bolt absorbs the cartridge's inertia until the bullet leaves the barrel.

Bolt Compatibility
- The rifle bolts on the HK G3 and CETME Model A aren't interchangeable without expert modification. Although the bolts operate the same way, minor design variations prevent compatibility. Both use the same roller and roller plates, but the HK bolt head is longer than the one on the CETME rifle.
Warning
- A rifle bolt is a site of extreme pressure, and changing the entire assembly can be difficult and dangerous. If the complete bolt carrier is replaced with one from a different rifle--even of the same model--the "headspace" may differ enough to make the weapon unsafe. This is the space between the closed breech of the gun and the surface where the cartridge sits. When replacing a rifle bolt carrier, the spacing must be checked before firing.
Considerations
- Compatible parts on the CETME Model A and HK rifles include the firing pin and firing pin spring, trigger assembly, recoil spring, and the cocking handle and spring.

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