A Modal analysis of a PHALANX gun assembly was conducted in the laboratory at Naval Postgraduate School. The goal of this analysis is to provide a dynamical description of the gun for studies of the effects of gun modifications on bullet dispersion. Accelerometer data were collected from five locations on the topmost barrel of the barrel assembly while the gun was being excited at the barrel tips with a swept sine forcing signal. This was done for five configurations of the gun, four of which did not include the production muzzle restraint, and the fifth that did. Frequency response data (vertical displacement/vertical force) were processed using the STARModal modal analysis software to extract modal parameters (frequencies, damping, and mode shapes) for frequencies up to 500Hz. For the four unrestrained configurations, there were no major differences in frequency response spectra, modal frequencies, or modal shapes. Eight modes were identified below 500Hz. For some modes the shape of the barrel assembly motion was very similar, suggesting that other structural responses might be present. The fifth configuration, with muzzle restraint, was distinct from the others in all categories, generally displaying lower modal frequencies and a higher modal frequency density. Thirteen modes were identified below 500Hz for this configuration. Again, there was some repetition of barrel assembly shapes for different modes. This study shows that the addition of the muzzle restraint, which was designed to reduce barrel tip vibration has resulted in a much higher modal density for frequencies above about 125Hz. This is significant, since it is known that the frequency spectrum of the forcing function of a firing bullet has significant amplitude above 125Hz.
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