Top: energy-loss time scales of cosmic-ray protons through secondary nuclear production (t_pp; solid curves) and of cosmic-ray electrons through bremsstrahlung (t_ff; dashed curves) are shown for parameters corresponding to type Ia and type II SNe, as labeled (values for type II SNe are given in parentheses in the legend). Also shown are time scales for Compton energy losses (t_C; dotted curves) of cosmic-ray electrons that scatter CMB photons to make γ rays of energy E_γ. Bottom: theoretical estimates of γ-ray luminosities of SNRs through secondary nuclear production (solid curves), bremsstrahlung (dashed curves), and Compton scattering (dotted curves). Data points show the measured >300 MeV γ-ray luminosities for the SNRs listed in Table 1. Thick curves in the two panels are for cosmic-ray interactions with the shocked external medium, and thin curves are for cosmic-ray interactions with the densest available target material. Model curves are plotted only in the weak-cooling regime.