September 9, 2011
I don’t know the technical term, so I just call it a "stop". When you’re taking the gun down, you unscrew the magazine tube and pull it out until the stop hits the retaining ring. Then twist the barrel assembly and remove. I guess it just prevents you from pulling it out too far.
October 27, 2012
It’s the magazine locking pin.
Push it in so it clears the barrel and use it as a lever to rotate the magazine tube so the pin is on the other side of the barrel. This disengages the interrupted threads of the tube in the action so the tube can be drawn forward and the barrel assembly rotated and removed. Reassemble in reverse order.
When assembled, the pin prevents the magazine tube from rotating and disengaging the interrupted threads.
February 22, 2014