Examination of externally moderated gas core reactor (GCR) neutronic calculations indicates that, in general, neutron diffusion theory is invalid and a higher order approximation to the transport equation needs to be employed. The Sn approximation yields accurate results but can require relatively long CPU computation times. A one-dimensional hybrid Sn-diffusion theory model is developed that employs the Sn approximation in the gas core region and for the first several mean free paths into the reflector region until the angular flux converges to its characteristic distribution in the reflector; diffusion theory is then used in the remaining portion of the reflector. A critical aspect of the hybrid scheme is to ensure proper interfacing between the Sn transport theory and diffusion theory approximations at the mathematical interface where the Sn-to-diffusion theory transition occurs. It is found that the point of transition from Sn theory to diffusion theory can be located closer to the core-reflector interface as the gas density in the core is reduced. Calculations performed on spherical GCR configurations for fuel gas densities ranging from 1018 to 1020 atom/cm3 and with both uniform and nonuniform fuel gas density distributions in the core show that the hybrid model gives accurate keff values and flux distributions as compared with results from the standard Sn approximation. For four energy groups and reflector thicknesses of 0.5 to 1.0 m, the hybrid model is roughly five times faster than a standard Sn calculation. For multigroup calculations on GCRs with thick (1 to 2 m) external moderator reflectors, the hybrid model is found to be about an order of magnitude faster than a standard Sn calculation.