  Coherent forward neutron propagation in gas is discussed as a new approach to
search for neutron-antineutron oscillations ($ n-\bar{n}$), which violate both
$B$ and $B-L$ conservation. We show that one can increase the probability of
neutron - antineutron transitions to essentially the free neutron oscillation
rate in the presence of a nonzero external magnetic field by tuning the density
of an appropriate mixture of gases so that the neutron optical potential of the
gas cancels that from an external magnetic field.
