I copied this from Remington re 870 extra barrels - 3" chambers.
"NOTE: 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."
This will take you to that page.
http://www.remington.com/sitecore/co...ls-tables.aspx