  Transformation of neutron to antineutron is a small effect that has not yet
been experimentally observed. %\cite{Phillips:2014fgb}. In principle, it can
occur with free neutrons in the vacuum or with bound neutrons inside the
nuclear environment different for neutrons and antineutrons and for that reason
in the latter case it is heavily suppressed. Free neutron transformation also
can be suppressed if environmental vector field exists destinguishing neutron
from antineutron. We consider here the case of a vector field coupled to $B-L$
charge of the particles ($B-L$ photons) and study a possibility of this to lead
to the observable suppression of neutron to antineutron transformation. The
suppression effect however can be removed by applying external magnetic field.
If the neutron--antineutron oscillation will be discovered in free neutron
oscillation experiments, this will imply limits on $B-L$ photon coupling
constant and interaction radius few order of magnitudes stronger than present
limits form the tests of the equivalence principle. If $n-\bar n$ oscillation
will be discovered via nuclear instability, but not in free neutron
oscillations in corresponding level, this would indicate to the presence of
fifth-forces mediated by such baryophotons.
