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Marc, Figuring your 870 Express Magnum is chambered in 12 ga. This barrel will fit and function. Great deer hunting barrel. Very accurate.

yes it will fit the 870 express mag

Marc, this will work but you would be limited to 3" Shells as your Magnum has a 3.5" Chamber. I would still confirm this directly with Remington before purchase at TEL: 1-800-243-9700

Marc, This is a great barrel. Will fit and function with your 870 Express Magnum. This barrel is $300 at my local shop. No problem with 2 3/4" slugs. . I shoot 2 3/4", 1 1/8th ounce loads out of my 3 1/2" chambered Super Magnum barrel and have yet to have a problem the odds of any issue are no greater than when shooting 2 3/4" shells from a 2 3/4" chamber. A semi-auto could be a different story if it is not inertia cycled due to SAAMI pressure for 2 3/4" and 3" shells is 11,500 psi; SAAMI pressure for the 3 1/2 " shell is 14,000 psi. Depending on the pressure/time curve, if you have powder burn-out before the gas port, some semi-automatics may malfunction due to insufficient pressure from 2 3/4" and 3" shells. Performance will be dependent on the amount of pressure a particular semi-automatic requires to function and the pressure generated as the wad/shot column passes the gas port. Pump action no problem.