The muzzle vibration response of a large-mass barrel gun is one of the key factors affecting firing accuracy. Considering the contact-collision interactions between mechanisms, this paper establishes a firing dynamics model for a prototype large-mass barrel gun. The influence of different neck tube lengths on muzzle vibration is investigated, and the relative sensitivity of various structural parameters to muzzle vibration is analyzed. The results show that extending the neck tube length from 1/4 to the full length of the barrel reduces vibration amplitude and decay time to 46% and 49.73% of their original values, respectively. Additionally, the stiffness of the elevation mechanism, the mass of the recoiling component, and the center-of-mass position significantly affect muzzle vibration. These findings provide valuable guidance for the overall structural design of large-mass barrel guns.