What is "WMR" and "convertibles"
WMR is a shortened term for 22 Magnum. Technically, the cartridge is 22 Winchester Magnum Rimfire. A convertible is a revolver that comes from the factory with 2 cylinders. You can shoot different cartridges with these guns. Ruger makes several single action type revolvers that are convertibles. One is the Single Six convertible. Its a 22 caliber rimfire Single Action revolver with 22 Long Rifle, AND 22 WMR cylinders. Since the bullet diameters of the 22 Long Rifle( and also 22Short and 22 Long) and the 22 Magnum are very close, you can shoot these cartridges thru a barrel that is a slightly compromised for one of the cartridges. In this case, the 22 Long Rifle will not achieve optimum accuracy, because the barrel is optimised for the larger diameter bullet( in this case, the 22 Magnum bullet, because it is slightly larger in diameter). Some people claim the Single Six is more accurate with 22 WMR than with 22 Long Rifle, but in practice, most people will never notice any difference. I have 2 Single Six convertibles, and both are very accurate with the 22 Long Rifle Cylinder. You simply remove the base pin, and then the cylinder, install the other cylinder, and reinstall the base pin. Its as simple as that. The reason you need 2 cylinders for the different 22 offerings is because the WMR cartridge is larger in diameter and if you loaded Long Rifle into the Magnum cylinder, the case will burst when fired.
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.
Since the original topic of this thread is about Rugers new Single Nine, the mention of people wanting it offered as a convertible with 2 Nine shot cylinders is a real possibility. The Single Six, Single Nine and Single Ten are all essentially the same guns, they share the same frames, and many parts are interchangable, so a Single Nine convertible is possible. The general consensus is that a Single Ten convertible wont work because there isnt enough room for ten 22 WMR rounds, thats why the Single Nine was born.