The shock sensitivities of plastic bonded explosives were studied with a thin flyer impact test by using two types of pressed RDX. The thin flyer, driven by an electrically exploding plasma, exerts a short-duration, high-pressure pulse to the samples to trigger a shock-to-detonation process. It was found that the duration and magnitude of the incident shock strongly influence the dominant mode of hot-spot formation, promoting a fast pore collapsing mechanism while suppressing other slower shear or friction mechanisms, as proposed by Chakravarty etal. [1]. The pressed PBX based on reduced sensitivity RDX had higher shock threshold pressure, compared to the pressed PBX based on commercial RDX. The difference was observed even with a certain portion of external extragranular defects. It is postulated that the internal crystal defects are more efficient than the external porosity in terms of the rapid reaction of hot spots.