That was my first shotgun. A beautiful action, but finding extra barrels was difficult. To be honest, I don't know if a rifled slug barrel even exists for 20ga Winchester pumps. They use the same barrels as 1300's BTW. You could always try a rifled choke tube, and use the bead for sighting (the 120 was ribless as I recall).
Or give a shot at trying a few various brands of rifled slugs through your smooth bore.
You may be pleasantly surprised at the accuracy you'll get with the right combo.
Extra nifty thing about a good slug outa your smoothbore is no added barrel cost & no having to switch barrels to get that all-round shooter - best of both worlds.
For the price of a barrel you may want to consider a H&R Ultra Slug Hunter it has a fully rifled heavy barrel comes with a sling and scope base and their dead accurate. I just bought one brand new for under $200.00 in 20ga.
