Delayed Blowback Firearms

Just for starters, Chinn made a distinction between "delayed" and "retarded" blowback guns:

- delayed blowback being guns in which the bolt is positively locked to the barrel at the instant of firing, with some mechanism (usually gas) unlocking it, then blowback doing the rest (20mm Hispano cannon is the best known example)

- retarded blowback being when the bolt is never locked, but its initial rearward movement is resisted by some means (mechanical or gas) until the gas pressure in the barrel overcomes it

According to this distinction, the vast majority of weapons which are popularly called "delayed blowback" are actually "retarded blowback".

It could be argued that another type of blowback is also "delayed": the API (advanced primer ignition) blowback, in which the gun is fired as the bolt is travelling forwards at high speed, so the chamber pressure first has to bring the bolt to a standstill before it can start pushing it back again. The WW2 Oerlikon guns are the best-known examples of this type.
 
The French FAMAS assault rifle uses a lever-delayed blow-back action.
 
HK MP 5 is a retarded blowback SMG, then HK P7, Norinco Type 77, M77 and NP-20 are gas retarded blowback pistols. :) Then the Gusloff VG 1-5 is a gas retarded blowback carbine.
And if I remember correctly, there was a conversion set for the M-1911 type pistols some 20-25 years ago, turning them into gas retarded blowback ones?
 
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