  We consider dynamical scales in magnetized GRB outflows, using the solutions
to the Riemann problem of expanding arbitrarily magnetized outflows (Lyutikov
2010). For high ejecta magnetization, the behavior of the forward shock closely
resembles the so-called thick shell regime of the hydrodynamical expansion. The
exception is at small radii, where the motion of the forward shock is
determined by the dynamics of subsonic relativistic outflows. The behaviors of
the reverse shock is different in fluid and magnetized cases: in the latter
case, even for medium magnetization, sigma ~ 1, the reverse shock forms at
fairly large distances, and may never form in a wind-type external density
profile.
