If I understood Karl Rohrbaugh correctly when we spoke by phone a few weeks ago, he worked with Corbon directly to develop a controllable 9mm round that would penetrate sufficiently and expand reliably when fired through a barrel as short as it is on the R9.
There was a fair amount of trial and error in the development of the round, and this, when combined with low demand (there is only one 9mm handgun out there with the characteristics of the R9), means a high per round price and limited availability (from Rohrbaugh).
I've ordered a few for carry rounds directly from Rohrbaugh, due in later this month.
For range use, Rohrbaugh recommends Federal's American Eagle 115g FMJ.