The change in ballistic performance of AISI 4340 steel plates with different thicknesses and varying hardness against 7.62 AP (Armor Piercing) projectiles is studied. Four different target plate hardnesses are chosen. The commercial hydrocode software ANSYS Autodyn is used in numerical studies. Johnson-Cook strength model parameters for the varying target plate hardness are calibrated from mechanical tests. The numerical simulation results using this model are confirmed by a series of experimental test results. Good agreement is obtained between the numerical results and experimental findings in terms of residual projectile velocities. Finally it is seen that the ballistic protection offered by AISI 4340 steel increases with the increasing hardness.