A comparison of this photograph with that of the closed one will disclose the distance of barrel'recoil by a comparison at the line of the rear sight.
At the instant of firing the barrel and breechblock are locked by the locking arms behind the breechblock being held firmly in place to support the breechblock-head. The unlocking flaps are hidden below the breechblock in this photo.
After short recoil during which pressure drops, the barrel hits its stop and is halted in rearward travel.
The forward ends of the locking arms are pushed outward, against the barrel extension by cam action of the short flaps as they turn and the breechblock continues to the rear to with the unlocked arms, indicated by the mottled steel tops, compress the recoil spring around its guide.
The photograph shows the breechblock in full rear position forced into their recesses in the barrel extension on either side of it.
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