Predictive modeling of bcc materials remains challenging due to the complex nature of slip activity, patterns and interactions. Additionally, the high-dimensional space of material parameters with significant uncertainties poses a challenge to the further development of continuum constitutive models especially at the single crystal level. In this work, we aim to identify key physical assumptions and the associated uncertainties for the predictive modeling of bcc single crystals under quasi-static to shock loading conditions. To this end, we employ two representative physics-based bcc single crystal plasticity models taken from our previous work (Nguyen et al., Int J Plasticity 139 (2021) 102940; Lee et al., Int J. Plasticity 163 (2023) 103529), each prioritizing different key deformation mechanisms. First, the Bayesian model calibration (BMC) is used for probabilistic estimates of parameters in both models using experimental over a wide range of strain rates, temperatures, and crystallographic orientations. In conjunction with the BMC procedure, the global sensitivity analysis is conducted to quantify the impact of uncertainties in the model parameters on the key simulation results of quasi-static to shock responses. The global sensitivity analysis of uniaxial stress responses at various strain rates and temperatures shows that the influence of individual parameters is strongly correlated with loading conditions. The sensitivity indices at various loading conditions clearly illustrate the physical basis underlying the predictive capabilities of the two distinct bcc models for the rate- and temperature-dependence. Then, both of the physics-based bcc crystal plasticity models are further validated at shock loading conditions beyond the calibration regime. By assessing the predictive capabilities of the two BMC-calibrated bcc models for plate impact experiments, we further identify critical physical mechanisms that govern the elastic-plastic transition in representative single crystal molybdenum under shock loading. Reference: https://arxiv.org/abs/2512.23985