A simple single-fluid model is presented which accounts for the behaviour of MHD shock producing devices in which the shock and the driving current sheet are unseparated. By means of both characteristic and similarity solutions, it is shown that unusual plasma behaviour is a simple consequence of the fluid equations of motion. The important parameters in determining the performance of MilD shock tubes are the width of the current sheet at breakdown (related to the magnetic Reynolds number) and the current sheet Mach number with respect to the gas at rest. It is shown that, under many conditions, separation is more likely to be achieved with a decrease, rather than an increase, in current sheet speed. Lowering the Mach number is more influential in causing the shock to form at the front of the current sheet than raising the magnetic Reynolds number.