  We revisit an alternate explanation for the turbulent nature of molecular
clouds - namely, that velocity dispersions matching classical predictions of
driven turbulence can be generated by the passage of clumpy material through a
shock. While previous work suggested this mechanism can reproduce the observed
Larson relation between velocity dispersion and size scale ($\sigma \propto
L^{\Gamma}$ with $\Gamma \approx 0.5$), the effects of self-gravity and
magnetic fields were not considered. We run a series of smoothed particle
magnetohydrodynamics experiments, passing clumpy gas through a shock in the
presence of a combination of self-gravity and magnetic fields. We find powerlaw
relations between $\sigma$ and $L$ throughout, with indices ranging from
$\Gamma=0.3-1.2$. These results are relatively insensitive to the strength and
geometry of magnetic fields, provided that the shock is relatively strong.
$\Gamma$ is strongly sensitive to the angle between the gas' bulk velocity and
the shock front, and the shock strength (compared to the gravitational
boundness of the pre-shock gas). If the origin of the $\sigma-L$ relation is in
clumpy shocks, deviations from the standard Larson relation constrain the
strength and behaviour of shocks in spiral galaxies.
