http://www.geminicustoms.com does a moonclip conversion for the Ruger SP101 .38 Special or .357 Magnum. The circumferance of the cylinder is left with the original surface so that individual cartridges can be loaded.Ruger Collector said:
Moonclips are also used with 9mm revolvers, such as the Ruger Speed Six and SP101, S&W 940? and I think Taurus may have made a model or two. Not relevant to the moonclip discussion but I recall Ruger also had an optional 9mm cylinder for its single action Blackhawk in .357 Magnum.
PS: I think I've seen a photo somewhere of a GP100 with a moonclip conversion.