Congrats on the new Shotty. Too bad about the Winny. Hope you can fix.
Anyway, swab out the barrel, and give other parts a coat of Oil, Rack Gun a half dozen times, wipe off excessive Oil, and shoot a couple Shots, swab out, give parts another coat of Oil/wipe down, Rack Gun 1/2 dozen times to help smooth things out, go and shoot 3 or 4 more shots, reswab, reoil, rack gun some more, wipe off, then go out and blast away.
One thing about the new 870. They don't like Short Stroking. They'll jam up every time. Be firm on your Pumps. After awhile, it'll be 2nd nature as to how the Express likes it. They are rough on the inside, but they do smooth out.