  Astronomical observations with gamma rays in the range of several hundred keV
to hundreds of MeV currently represent the least explored energy range. To
address this so-called MeV gap, we designed and built a prototype CsI:Tl
calorimeter instrument using a commercial off-the-shelf (COTS) SiPMs and
front-ends which may serve as a subsystem for a larger gamma-ray mission
concept. During development, we observed significant non-linearity in the
energy response. Additionally, using the COTS readout, the calorimeter could
not cover the four orders of magnitude in energy range required for the
telescope. We, therefore, developed dual-gain silicon photomultiplier (SiPM)
boards that make use of two SiPM species that are read out separately to
increase the dynamic energy range of the readout. In this work, we investigate
the SiPM's response with regards to active area ($3\times3 \ \mathrm{mm}^2$ and
$1 \times 1 \ \mathrm{mm}^2$) and various microcell sizes ($10$, $20$, and $35
\ \mu \mathrm{m}$). We read out $3\times3\times6 \ \mathrm{cm}^3$ CsI:Tl chunks
using dual-gain SiPMs that utilize $35 \ \mu \mathrm{m}$ microcells for both
SiPM species and demonstrate the concept when tested with high-energy gamma-ray
and proton beams. We also studied the response of $17 \times 17 \times 100 \
\mathrm{mm}^3$ CsI bars to high-energy protons. With the COTS readout, we
demonstrate a sensitivity to $60 \ \mathrm{MeV}$ protons with the two SiPM
species overlapping at a range of around $2.5-30 \ \mathrm{MeV}$. This
development aims to demonstrate the concept for future scintillator-based
high-energy calorimeters with applications in gamma-ray astrophysics.
