Body armor prevents projectile penetration, but thoracic visceral injury may still result from pressure-wave transmission and back-face deformation. However, thoracic responses to single and multiple impacts remain insufficiently understood, numerical human thoracic models need further validation, and load transfer between the rib cage and cardiopulmonary system remains unclear. Therefore, this study combined live-fire blunt-impact tests on a biomimetic thoracic target with a human thoracic finite element model with filled thoracic cavity gaps. Load transfer and cardiopulmonary responses under non-penetrating rifle-bullet impact were investigated using a SiC/UHMWPE composite ballistic insert. The results showed that, first, under a single impact, the right-lung pressure fluctuated rapidly in the initial stage, then entered an oscillatory process and gradually tended to stabilize; under multiple impacts, the peak right-lung pressure exhibited nonlinear fluctuation characteristics. Second, after the thoracic visceral organs were encapsulated with soft-tissue materials, the model was validated, and the continuity of internal load transfer was significantly enhanced. Third, during the initial establishment stage of back-plate deformation, the displacement responses of the sternum, costal cartilage, and cardiopulmonary tissues could be described by a linear–exponential function; after significant compression of the back plate occurred, the stress responses of the rib cage, right lung, and heart conformed to the characteristics of a power–exponential composite function. Finally, the pressure peak on the posterior side of the sternum was only 39.3% of that on the impact-facing surface; relative to the rib cage, the stress peaks in the right lung and heart were attenuated by 65.3% and 82.2%, respectively, indicating that the protection of cardiopulmonary visceral organs by the rib cage was mainly achieved through the synergistic effect of rigid shielding by the sternum and flexible energy dissipation by the costal cartilage. The research results can provide references for the back-plate stiffness design of ballistic inserts, optimization of buffer-layer structures, and assessment of thoracic blunt injury risk.