A gun preferably designed to rapidly fire paint balls includes a single valve assembly that is inserted into and removed from the gun body. The valve assembly includes a low pressure ram passing through a longitudinal center or core of the assembly, and is capable of being driven in opposite reciprocal directions. A volumizer stores a charge of high pressure gas sufficient to adequately propel a paint ball, and a high pressure poppet valve is driven by movement of the low pressure ram for releasing the charge rapidly. A bolt carried upon the low pressure ram moves a paint ball into firing position just prior to the high pressure discharge. All of the aforementioned components of the cartridge are carried upon a single axis defined by the ram. Means are provided to both align and couple the various components together to ensure proper operation at high firing rates.