I built a .62 cal longrifle for a friend about thirty years ago, and just a few years ago, he wanted me to add a 20 gauge smooth barrel to the rifle so he could shoot both smoothbore matches and shot. I had John Getz recreate the barrel in 20 ga. smooth, and filed a tang out of a length of 1 1/4" bar stock. Worked out very well.
Daryl has a rifle that I believe has three barrels: .40, .45, and .50 cal.