For the record, the kg/m^2 precip units used in GRIB for all NCEP models, including the RUC, does convert to mm, as Phil Harvey confirmed.

The RUC2-based MOS/FOUS web page that Tom Hicks developed is really nice, and there is now a link to it from the RUC homepage (scroll down in the main page to "Other Places to Get A Piece of the Operational RUC"). Take a look at it -- it has MOS/FOUS-type messages from the RUC for 42 different locations, and you also can look for consistency between recent runs.

Stan