Pretty sure any Winchester 1200/1300 barrel with fit the 120 but maybe someone will chime in with a definitive answer. Yes, a factory original 18" bbl. will likely be hard to find...heck, any bbl. that will fit it may be hard to find and/or expensive. You may wind up having to buy a longer one and cut it, or cut one of the two you have, if you're determined to go with 18". The new Winchester Super-X pump gun also looks like the same gun as the 1300, just a different marketing program. Again, someone here may know if it is.