  Moment expansions are used as model reduction technique in kinetic gas theory
to approximate the Boltzmann equation. Rarefied gas models based on so-called
moment equations became increasingly popular recently. However, in a seminal
paper by Holway [Phys. Fluids 7/6, (1965)] a fundamental restriction on the
existence of the expansion was used to explain sub-shock behavior of shock
profile solutions obtained by moment equations. Later, Weiss [Phys. Fluids 8/6,
(1996)] argued that this restriction does not exist. We will revisit and
discuss their findings and explain that both arguments have a correct and
incorrect part. While a general convergence restriction for moment expansions
does exist, it cannot be attributed to sub-shock solutions. We will also
discuss the implications of the restriction and give some numerical evidence
for our considerations.

Fluid Dynamics;Computational Physics