Removing the Extended Barrel on the Hawsan Mad Max Shotgun

[Taylor7500]

Just wondering if anyone has any insight on how to do this. I have the extended version of the gun which I've been told can have the extended barrel removed (non-permanently) to effectively convert it into the sawn-off version and back. But I don't know how.

 

I've taken the four little grub screws out (two at the ends holding the rubber caps in place, two in the middle) and removed the inner barrels. After that I can't see anything which would be holding the two barrel pieces together but they just won't budge. No movement at all. I've given it a reasonable amount of force, I've tried heat, and there's no give at all, as though there's still a pin or screw somewhere holding it in place.

 

I don't suppose anyone with experience doing this could tell me how to do this?