I think that the M 17 and M31 take down is very similar. There is a knurled ring at the front of the magazine tube that must be turned to disengage the barrel from the magazine tube before the barrel can be rotated 90 degrees to remove it. The lug on the barrel has a pin that engages a hollow pin in the magazine cap. The hollow pin is threaded into the magazine cap and retracts to free the pin on the barrel lug when the knurled ring on the magazine cap is turned. Once the barrel is off, a screw through the band around the magazine has to be removed to remove the magazine cap and install the plug.
