This article reports on work to develop and demonstrate a diagnostic technique using infrared fiber optics to probe the decomposition processes in burning gun propellant strands. The experiments reported here involve measuring the absorption across an open gap between two embedded fibers as it fills with gaseous decomposition products. Spectroscopic detection is achieved using pairs of bandpass filters. The absorption record can be correlated with readings from embedded thermocouples and with a high-resolution video recording of the burn. We have observed N2O, a decomposition product of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) evolving into the observation volume. The N2O appearance is often an abrupt event. Coupled with evidence that it can occur while the observation region is relatively cool and far from the burning surface (observed in cases involving burning at 6 atm pressure), these observations suggest hypotheses concerning the physical processes in the condensed phases of this propellant, hypotheses including the development of pressurized voids or matrix-trapped bubbles, which can release decomposition gases into the unburnt propellant through cracks that suddenly open. The appearance of N2O after the temperature record suggests that melting and decomposition of RDX have begun (observed in I-atm cases), implies that N2O formation is indeed not an initial step in RDX decomposition under our conditions.