<p>The basic problem of the armor steel joint is that the superior mechanical properties and the ballistic performance of the weld metal of the joint are partially lost due to weld thermal cycling. In this study, the ballistic performance of the automated gas metal arc-welded (GMAW) high-hardness Bisalloy HHA500 armor steel with as-welded and post-weld heat-treated conditions was investigated. The microstructure investigations of the joints were carried out. Ballistic tests were performed to determine the ballistic resistance of the weld metal and HAZ and ballistic penetration in the weld zone, which is defined as the weak zone in the ballistic literature. Results confirm that ballistic test results for 5.56 × 45 NATO SS109 and 5.56 × 45 M193 rounds fired with NATO STANAG 4569 Level 1 ammunition were successful for all as-welded and PWHT samples. The changes in the microstructure affect the hardness of the weldment, particularly in the HAZ region near the base metal, where softening occurs. This softening leads to weakened ballistic performance in this region. Although there is not a great difference in the ballistic test results depending on the post-weld heat treatment, all heat-treated joint tests were successful except for the damage of the 7.62 × 51 NATO Ball (Ball M80) bullet with high muzzle energy used in the tests.</p>