An injected round bore railgun (77 cm long by 1 cm diameter bore) was designed and built for testing insulator materials. The firing parameters were held fixed and the rail to rail resistance of various insulators was measured versus shot number. The insulators studied include a variety of glass filled polymers, and filled and unfilled thermoplastics. Insulator, projectile, and rail mass losses were also measured. A voltage polarity dependence of the rail mass loss was clearly demonstrated. The surface conduction of the insulators was measured as a function of distance from the breech. The tests indicate that a significant improvement in performance over filled epoxy materials is possible with certain polymers.