  We present observations and modeling of SN 2016hnk, a Ca-rich supernova (SN)
that is consistent with being the result of a He-shell double-detonation
explosion of a C/O white dwarf. We find that SN 2016hnk is intrinsically red
relative to typical thermonuclear SNe and has a relatively low peak luminosity
($M_B = -15.4$ mag), setting it apart from low-luminosity Type Ia supernovae
(SNe Ia). SN 2016hnk has a fast-rising light curve that is consistent with
other Ca-rich transients ($t_r = 15$ d). We determine that SN 2016hnk produced
$0.03 \pm 0.01 M_{\odot}$ of ${}^{56}\textrm{Ni}$ and $0.9 \pm 0.3 M_{\odot}$
of ejecta. The photospheric spectra show strong, high-velocity Ca II absorption
and significant line blanketing at $\lambda < 5000$ Angstroms, making it
distinct from typical (SN 2005E-like) Ca-rich SNe. SN 2016hnk is remarkably
similar to SN 2018byg, which was modeled as a He-shell double-detonation
explosion. We demonstrate that the spectra and light curves of SN 2016hnk are
well modeled by the detonation of a $0.02 \ M_{\odot}$ helium shell on the
surface of a $0.85 \ M_{\odot}$ C/O white dwarf. This analysis highlights the
second observed case of a He-shell double-detonation and suggests a specific
thermonuclear explosion that is physically distinct from SNe that are defined
simply by their low luminosities and strong [Ca II] emission.
