Kind of like a muzzle break, but it actually uses the burned gas pressure to push back on the barrel muzzle, which cycles the lock, advances the belt, etc. The barrel actually travels fore and aft with each shot.
There is also a "fusee" spring on the left side that can be adjusted for tension which will make the gun cycle faster or slower.
In the winter they had to fire a few shots to get things working properly.
dju
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