  Massive gravity (mGR) is a 5(=2s+1) degree of freedom, finite range extension
of GR. However, amongst other problems, it is plagued by superluminal
propagation, first uncovered via a second order shock analysis. First order mGR
shock structures have also been studied, but the existence of superluminal
propagation in that context was left open. We present here a concordance of
these methods, by an explicit (first order) characteristic matrix computation,
which confirms mGR's superluminal propagation as well as acausality.
