Virginian-in-LA says you need a 20 gauge light weight (lw) magnum barrel. IF THAT IS TRUE, there is a 28" vent rib barrel marked Magnum 3" on Ebay with a set to close bidding @ 11:40 PDT today.
http://www.ebay.com/itm/Remington-Mo...item43b2082c17
That may be a fixed choke barrel.