The intermediate flash region that develops downstream from the Mach disk during the discharge of a 7.62-mm rifle into ambient air is examined by laser-Doppler velocimetry. The objective is to investigate whether or not slip surfaces are formed which would prevent the entrainment of oxygen and, therefore, the initiation of combustion reactions in the intermediate flow region. In addition, local temperature histories are measured in this flow region during the discharge of the rifle into air, nitrogen, and oxygen using specially designed, intrusive temperature gauges. Here, the objective is to examine whether or not oxygen is entrained and induces combustion processes in the intermediate flash region. Then local temperatures should increase with the oxygen content of the surroundings. It has been found from velocity data that slip surfaces are formed in the first few millimeters and then are absent. Also, the observed temperatures are ranked in the order T(N2) < T(air) < T(O2). It follows that oxygen is entrained, and combustion reactions take place in the core of the intermediate flow region. These combustion reactions control the ignition sequence leading to the secondary flash, and they are inhibited by alkali salts.