Ejecta masses M_ej vs. CSM masses M_CSM of our Type IIn supernova progenitors (main panel). The right and upper panels show the M_CSM and M_ej histograms. The lines in the scatter plot indicate constant f_M (Eq. 2), which represents the conversion efficiency of the ejecta kinetic energy into radiation as it impacts the CSM, assuming spherical symmetry. We distinguish the contribution from models undergoing stable Case C (filled orange), unstable Case C (filled blue), and Case C following Case B (empty orange). The inferred properties from a sample of observed Type IIn supernovae from Ransome & Villar (2025) is also shown (gray dots).