Ammonium bromide undergoes a pressure-induced order-disorder transition, where the disordered (plastic) phase is thought to consist of a random distribution of ammonium cations that can freely rotate between two preferred orientations - an Ising-like model. Spectroscopic and diffuse scattering measurements have suggested the presence of short range order (SRO) in the plastic phase, that clearly must propagate over an 'infinite' range at the transition point to the ordered crystalline solid above 2.3 GPa. Our experiment aims to monitor the development of long range order from SRO within the plastic phase as the transition is approached and model this using the reverse Monte Carlo method. The use of argon gas as a pressure transmitting medium means the total scattering pattern can be measured and corrected under quasi-hydrostatic conditions.