The Ultras and Handi centerfire barrels are interchangable, they will fit all SB2 frames except the 10ga frame. New barrels are only available fitted from the factory, but many here have bought used barrels and usually they will fit, if not they can be fitted with a little work. Most of us have more than one frame so getting a barrel to fit one frame or another helps. SB2 frames are heat treated and much stronger than the SB1 shotgun and SS1 rimfire frames.
Tim