Materials with large caloric effects offer the opportunity to replace current refrigeration technologies that require the compression of hazardous gases. Whilst magnetocaloric materials driven using magnetic fields are currently the most developed for applications, barocaloric materials driven using hydrostatic pressure are advantageous due to the relative ease and cost of creating mechanical pressure compared to magnetic field. Our recent pressure-dependent magnetisation and calorimetry measurements on the Mn antiperovskite Mn3NiN have revealed a giant barocaloric effect near its first-order paramagnetic to antiferromagnetic transition. Here we propose to use HRPD to determine the magnetic and nuclear structure of this material, as a function of temperature and pressure, in order to understand the origins of our observed giant barocaloric effect.