  The effects of pressure up to $\sim 20$ kbar, on the structural phase
transition of lightly Sn-doped BaFe$_2$As$_2$, as well as on the
superconducting transition temperature and upper critical field of
(Ba$_{0.55}$K$_{0.45}$)Fe$_2$As$_2$ single crystals have been studied. Both
transitions decrease with pressure in an almost linear fashion. Under pressure,
the upper critical field curve, $H_{c2}(T)$, shifts down in temperature to
follow the zero field $T_c$ with very little change in slope.
