To study the dynamic performance of a grenade machine gun system with soft recoil technology and analyze the floating mechanism parameters affecting the firing dispersion, a rigid-flexible coupling multi-body dynamics model was established. A twenty-round firing process of the grenade machine gun system was simulated and analyzed. The firing dispersion of the grenade machine gun system was obtained based on the external ballistics model, and the simulation model was validated by comparing the numerical results with the experimental data. The effects of the floating mechanism parameters on the cradle force, the projectile dispersion, and the firing dispersion sparsity were discussed. The results show that the projectile dispersion of the GMG system is well predicted by the simulation model, and the floating parameters that are matched reasonably could improve the firing dispersion and reduce the gun recoil force.