On a G.Gibbs takedown the front screw, connecting magazine assembly to receiver ring, functions a the "catch". It has a cone-shaped point that goes through the receiver ring into a recess bored into the barrel threads, locking the barrel. To take down, you remove the forend, unscrew this screw a few turns. Then you simply unscrew the barrel. Here such a Gibbs Mannlicher is shown disassembled. In some of the photos you can see the backed-out screw.
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