The 18in 500 barrel will fit---just shoot no bigger than 3in shells when using it.
If you want the 20in--you'll have to replace the mag tube with somthing like this:
http://www.brownells.com/.aspx/pid=2...TY-SHOTGUN-KIT
AGAIN--USE NO LARGER THAN 3IN SHELLS WHEN USING THIS
But then your hunting barrel won't fit on this mag tube.