Slice through the shock model showing the x-velocity from the Enzo simulation described in Sect. 3.3 at t = 100 yr. The velocities correspond to motions along the line-of-sight. Owing to the nature of the simulation, since the bullet is initialized at rest the medium is flowing past it, small x-velocities signify material that has not been significantly accelerated. Or, when frame-shifted to the case of a moving bullet, material that has not been significantly de-accelerated.