Limits on the neutron-antineutron oscillation time from the stability of   nuclei
  We refute a recent claim by Nazaruk that the limits placed on the free--space neutron--antineutron oscillation time $\tau_{{n\bar n}}$ can be improved by many orders of magnitude with respect to the estimate $\tau_{{n\bar n}}>2(T_0/\Gamma)^{1/2}$, where $T_0$ is a measured limit on the annihilation lifetime of a nucleus and $\Gamma\sim 100$ MeV is a typical antineutron-nucleus annihilation width. 
C. B. Dover, A. Gal and J.M. Richard