Here's how to disassemble the HD model with the extension:
Unscrew and remove the screw in the side of the barrel clamp, then slide the clamp forward and off the gun.
The extension tube is screwed into a collar or coupling.
Unscrew the extension tube from the collar, LEAVING THE COLLAR IN PLACE.
WARNING: The magazine spring is LONG and VERY strong. As soon as the extension tube is unscrewed, the spring will try to "blow" out of the gun.
As you unscrew the extension tube, HOLD ON TIGHT, and allow the tube to move up JUST enough to allow you get your fingers around the spring.
Remove the extension tube, then carefully allow the spring to expand out of the magazine.
Remove the magazine spring.
Open the action about 1/2 way, then unscrew the coupling or collar from the front of the gun's magazine tube.
With the coupling off, pull the barrel forward , and off the gun.
Follow the owner's manual for the rest of disassembly.