Doug.
If you look at image 2 you will notice that protruding from the forend is a spring loaded fastener ,the underside of the barrel at this given point has a hook on the underside .When the barrel is placed into the hooked breech the barrel is pressed down which pushes the spring loaded fastener to one side and springs back to secure the barrel.
Has for the barrel yes it is reduced in size by the band in the middle of the barrel.Will try and take more images showing the fastener of the barrel
Feltwad