The HIBEAM/NNBAR experiment is a free-neutron search for $n rightarrow$ sterile $n$ and $n rightarrow bar{n}$ oscillations planned to be installed at the European Spallation Source under construction in Lund, Sweden. A key component in the experiment is the detector to identify $n-bar{n}$ annihilation events, which will produce on average four pions with a final state invariant mass of two nucleons, around $1.9,$GeV. The beamline and experiment are shielded from magnetic fields which would suppress $n rightarrow bar{n}$ transitions, thus no momentum measurement will be possible. Additionally, calorimetry for particles with kinetic energies below $600,$MeV is challenging, as traditional sampling calorimeters used in HEP would suffer from poor shower statistics. A design study is underway to use a novel approach of a hadronic range measurement in multiple plastic scintillator layers, followed by EM calorimetery with lead glass. A prototype calorimeter system is being built, and will eventually be installed at an ESS test beam line for textit{in situ} neutron background studies.
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