From the Remington website: Model 870 Extra Barrels with 3" chambers will fit Super Magnum, Magnum, and 2-3/4" receivers. These barrels will handle both 2-3/4" and 3" shells when used on Magnum receivers. Use only 2-3/4" shells when these barrels are used on 2-3/4" receivers.
Cabela's Canada has a fully rifled 870 barrel: https ://www.cabelas.ca/index.cfm?pageID=79§ion=&ID=4669