The low-temperature plasticity of body-centered cubic (bcc) metals is governed by 1/2 111 screw dislocations due to their compact, non-planar core. It has been proposed that 70.5 degrees mixed (M111) dislocations may also exhibit special core structures and comparably large Peierls stresses, but the theoretical and experimental evidence is still incomplete. In this work, we present a detailed comparative study of the M111 dislocation in five bcc transition metals on the basis of atomistic simulations. We employ density functional theory and semi-empirical interatomic potentials to investigate both the core structure and the Peierls barrier of the M111 dislocation. Our calculations demonstrate that reliable prediction of M111 properties presents not only a very stringent test for the reliability of interatomic potentials but is also challenging for first-principles calculations for which careful convergence studies are required. Our study reveals that the Peierls barrier and stress vary significantly for different bcc transition metals. Sizable barriers are found for W and Mo while for Nb, Ta and Fe the barrier is comparably small. Our predictions are consistent with internal friction measurements and provide new insights into the plasticity of bcc metals.