Just like any shotgun. Pull down on the front end of the forend until it snaps off, then open the action until the barrel unhooks from the pivot pin. These often seem to be sticky opening after they've been sitting unused for a while. It seems that the ejector mechanism always needs a hefty dose of penetrating oil and a good cleaning.
(If you removed the forend by undoing the screw, put it back, then follow instructions above.)
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