  The $\gamma$-process nucleosynthesis in core-collapse supernovae is generally
accepted as a feasible process for the synthesis of neutron-deficient isotopes
beyond iron. However, crucial discrepancies between theory and observations
still exist: the average production of $\gamma$-process yields from massive
stars are too low to reproduce the solar distribution in galactic chemical
evolution calculations, and the yields of the Mo and Ru isotopes are by a
further factor of 10 lower than the yields of the other $\gamma$-process
nuclei. We investigate the $\gamma$-process in 5 sets of core-collapse
supernova models published in literature with initial masses 15, 20, and 25
M$_{\odot}$ at solar metallicity. We compared the $\gamma$-process
overproduction factors from the different models. To highlight the possible
effect of nuclear physics input, we also considered 23 ratios of two isotopes
close to each other in mass, relative to their solar values. Further, we
investigated the contribution of C-O shell mergers in the supernova progenitors
as an additional site of the $\gamma$-process. Our analysis shows that a large
scatter among the different models exists for both the $\gamma$-process
integrated yields and the isotopic ratios. We found only 10 ratios that agree
with their solar values, all the others differ by at least a factor of 3 from
the solar values in all the considered sets of models. The $\gamma$-process
within C-O shell mergers mostly influence the isotopic ratios that involve
intermediate and heavy proton-rich isotopes with $\rm A>100$.
