Don't use steel shot in your .870 mod choked gun.Or you will score[damage the barrel].You can buy another barrel with REM CHOKES and use steel shot.A 3" chambered barrel with fit and work on a 2 3/4" frame BUT SHOOT ONLY 2 3/4 SHELLS IN IT! YOU NEED A MAGNUM 3" FRAME TO SHOOT 3" SHELLS! Confirm this by contacting Remington Arms to be safe. It may be cheaper to just buy a new 870 Express Magnumwith the correct barrel? Good Luck ............Harold