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The interaction of relativistic magnetized ejecta with an ambient medium isstudied for a range of structures and magnetization of the unshocked ejecta. Weparticularly focus on the effect of the ambient medium on the dynamics of animpulsive, high-sigma shell. It is found that for sufficiently high values ofthe initial magnetization $\sigma_0$ the evolution of the system issignificantly altered by the ambient medium well before the shell reaches itscoasting phase. The maximum Lorentz factor of the shell is limited to valueswell below $\sigma_0$; for a shell of initial energy $E=10^{52}E_{52}$ ergs andsize $r_0=10^{12}T_{30}$ cm expelled into a medium having a uniform density$n_i$ we obtain $\Gamma_{\rm max}\simeq180(E_{52}/T_{30}^3 n_i)^{1/8}$ in thehigh sigma limit. The reverse shock and any internal shocks that might form ifthe source is fluctuating are shown to be very weak. The restriction on theLorentz factor is more severe for shells propagating in a stellar wind.Intermittent ejection of small sub-shells doesn't seem to help, as the shellsmerge while still highly magnetized. Lower sigma shells start deceleratingafter reaching the coasting phase and spreading away. The properties of thereverse shock then depend on the density profiles of the coasting shell and theambient medium. For a self-similar cold shell the reverse shock becomes strongas it propagates inwards, and the system eventually approaches the self-similarsolution derived recently by Nakamura \& Shigeyama.

Interaction of a magnetized shell with an ambient medium: limits on impulsive magnetic acceleration