  High quality HgCr$_2$Se$_4$ single crystals have been investigated by
magnetization, electron transport and Andreev reflection spectroscopy. In the
ferromagnetic ground state, the saturation magnetic moment of each unit cell
corresponds to an integer number of electron spins (3 $\mu_B$/Cr$^{3+}$), and
the Hall effect measurements suggest n-type charge carriers. Spin polarizations
as high as $97\%$ were obtained from fits of the differential conductance
spectra of HgCr$_2$Se$_4$/Pb junctions with the modified
Blonder-Tinkham-Klapwijk (BTK) theory. The temperature and bias-voltage
dependencies of the sub-gap conductance are consistent with recent theoretical
calculations based on spin active scatterings at a superconductor/half metal
interface. Our results suggest that n-HgCr$_2$Se$_4$ is a half metal, in
agreement with theoretical calculations that also predict undoped
HgCr$_2$Se$_4$ is a magnetic Weyl semimetal.
