Wait a second. All Expresses to my knowledge are chambered for 3 inch. 3 inch chambers became standard on 870's in 1985. The barrel warning is for what Whiskeysnoot mentioned, don't put the barrel off your express on an older Wingmaster that isn't marked 'magnum' and shoot 3 inch shells.

Blast away with those 3 inchers, buddy! Oh, and edit out your message with some bleeps.