The mechanical behavior of polymers at ultrahigh strain rates can be very different from their typical behavior because of the inertia effect, wave propagation, adiabatic processes, and rate-dependent polymer chain dynamics. Polymer chain entanglements play a significant role in the mechanical properties of polymers. By employing microscopic ballistic testing in vacuum, the high-strain-rate intrinsic behavior of polystyrene (PS) is demonstrated depending on its molecular weight and microprojectile's perforation speed. Based on the experimentally quantified correlation between the specific penetration energy and the entanglement density, we predict the upper limits of the specific penetration energy of PS.