To get the barrel off, the mag cap/mag extension has to be removed.
If just a mag cap, unscrew & pull barrel off.
If extension without clamp, unscrew extension & pull barrel off.
If extension with clamp, can't get either the extension or the barrel off without removing the clamp that...clamps them together.
Remove clamp, unscrew extension, pull barrel off.
Be ready for the mag spring to push the extension out of your hand when the extension clears the threads.
The normal mag cap doesn't involve that, it should have a stamped steel internal "cap" inside the tube that retains the mag spring.
Denis