In recent years, there have been significant strides made in modeling gun dynamics. Current capabilities include the use of flexible gun tube and projectile descriptions. However, modeling the interface conditions between the tube and projectile is still an area of very active research. Highly detailed finite element models, which are currently under development at the BRL and at other agencies will lead to a much better understanding of the important parameters involved in the balloting of projectiles. These models, however, require both substantial computer resources and time. Thus there is a need for the development of simplified tube/projectile interface descriptions which both realistically represent the interface and are easily implemented. The bourrelet/ gun-bore and obturator models incorporated in this study reasonably satisfy these requirements. A nonlinear dynamic analysis of a balloting projectile is presented in this report. The mathematical model used is a six-degree-of-freedom coupled model of the projectile and gun tube system. The effects of obturator flexibility and projectile impact with the gun bore at the bourrelet are included in the analysis, and the nonlinear differential equations of motion of the system are derived using a Lagrangian formulation. Gaussian elimination and Newark's constant-average-acceleration method are employed to obtain a solution. Finally, numerical results for some specific test cases are obtained and discussed. Keywords: Computer simulation; Computer models; Interior ballistics.