This work, by means of the numerical modeling of a two-stage light gas gun, analyses deterioration parameters and proposes a procedure for diagnosing functional parameters of two-stage light gas guns with high firing frequency. Analyzed parameters are: piston wear and deformation, delay in first-stage valve opening, and piston seize-up. The procedure was developed by means of gas-dynamic numerical modeling. Experimental tests demonstrate that this procedure can be realistically applied.