  We here re-examine the validity of the constant-index power-law relation
between the fireshell Lorentz gamma factor and its radial coordinate, usually
adopted in the current Gamma-Ray Burst (GRB) literature on the grounds of an
"ultrarelativistic" approximation. Such expressions are found to be
mathematically correct but only approximately valid in a very limited range of
the physical and astrophysical parameters and in an asymptotic regime which is
reached only for a very short time, if any.
