  The fastest ejecta of supernova explosions propagate as a precursor to the
main supernova shock wave, and can be quite energetic. The spectrum of such
fast ejecta is estimated based on recent analytic and numerical supernova
models, and found to be a power law having a cutoff at an energy of order 10
MeV/nucl, the precise value of which depends on the supernova mass and
energetics. With cutoffs in this range there can be significant flux with
energies above the thresholds for $\gamma$-ray and Li, Be, and B production.
These nuclear interactions should inevitably accompany the passage of prompt
supernova ejecta through the surrounding medium. The fast particle composition
is that of the outermost layers of the progenitor; if the progenitor
experienced significant loss of its envelope to a companion or by mass loss,
then the composition is nonsolar, and in particular, metal-rich. Such a
composition and spectrum of fast particles is required to explain the recent
COMPTEL observations of $\gamma$-ray emission in Orion. Supernova ejecta from
one progenitor having lost a large amount of mass are shown to quantitatively
account for the Orion $\gamma$ rays, and to imply the presence of other, weaker
lines and of a supernova explosion in Orion in the last $\la 10^4$ years.
Implications for this mechanism as a mode of nucleosynthesis, and as a
potential supernova diagnostic, are discussed. Model dependences and
uncertainties are noted, and the need is shown for accurate measurements of
$\gamma$-ray and spallation production cross sections at energies near
threshold.
