Rimless rounds aren't a problem in a single action. Instead of headspacing on the rim, they headspace on the case-mouth.
On many types of rimless-cartridge revolvers (Ruger 9mm SP101, for example), the rounds will load just fine without moonclips. In those revos, the cylinder is reamed to fit the round, and the cartridge headspaces on the case mouth. However, if you don't use moonclips, you have to use your fingernail or a pencil to eject the brass, as the extractor has no rim or moonclip to grab.
Since a Ruger SA ejects brass with an ejector-rod which slides into the brass, there's no extractor, ergo rim is not necessary.
The downside of headspacing on the case-mouth is that only the exact round the cylinder is bored for will fit. No 38Spl/357Mag interchangeability with rimless rounds.
Hope that clears that up.