Originally Posted by weblance
There are other convertibles available, Ruger makes several BlackHawks that ship with 2 cylinders, a .357-9mm, and a 45 Long Colt-45 ACP. There are also Double Action 22 revolvers that are convertible. They are the Taurus 922 and Charter Arms Pathfinder Combo. These are both 22 Long Rifle-22 WMR. There may be others also, but these are in current production.
For what it's worth, the 9mm and .357 are not exactly the same diameter either, .355 vs .357, but close enough to work.