<p>This paper systematically investigates the penetration behavior of composite-structured projectiles (7075 aluminum alloy sleeve and 30CrMnSiA steel core) into the 921A steel targets at high velocities (500–1400&#xa0;m/s). Ballistic experiments were conducted to validate the model, and appropriate simulations were performed to extend the findings. Combining experimental and simulation results, penetration exhibits a coupled failure mode combining radial expansion and plugging, forming petal-like edges (impact surface) and bulging/plugging (rear surface), with perforation diameters ranging from 1.3<i>d</i>₁ to 1.7<i>d</i>₁ (<i>d</i>₁: projectile diameter). The ballistic limits (<i>V</i>₅₀) were quantified, demonstrating an inverse correlation with projectile mass. The residual velocity growth rate decreases with increasing initial velocity. Furthermore, the aluminum sleeve reduces <i>V</i>₅₀ by 2.7–4.1% while expanding perforation diameters by 8.9–45.7%, and the maximum perforation expansion ratio typically occurs within the 1.1–1.2<i> V</i>₅₀ range, with its effects diminishing at velocities exceeding1.5<i>V</i>₅₀. Extreme conditions (strain rates &gt; 10⁵&#xa0;s⁻¹, Δ<i>T</i> &gt; 1700&#xa0;K) induce thermal softening, melting, and microstructural transformations (austenitization, martensite formation, carbide dissolution), critically altering failure modes. These results advance the understanding of thermo-mechanical coupling in penetration resistance, providing a foundation for optimizing armor materials under high-strain-rate impacts.</p>