Pulsed plasma guns are used to obtain high-velocity (107–108 cm/s) plasma flows. Their performance is restricted by an instability of the plasma acceleration by a magnetic field. This paper presents results of a 2D numerical study of plasma dynamics in the plasma gun. The ZENIT-2D code solving the magnetohydrodynamic (MHD) equations on a fixed Eulerian mesh is used. The plasma parameters and geometry are chosen to be close to the parameters of the MK-200 installation (Sidnev et al., 1983). The influence of the initial distribution of a neutral gas on accelerator performance is investigated. A brief description of the code and details of the simulations are presented. It is shown that the instability of acceleration leads to turbulent mixing of the plasma and magnetic field and, correspondingly, to a broader current channel than that predicted by the classical diffusion with the Spitzer conductivity. Numerical results are compared with experimental data (Bakhtin & Zhitlukhin, 1998) displaying a good qualitative agreement.