AR-500 is extreme overkill for handguns. Most handgun targets are made from 3/8"
A36 mild steel.
A572 Grade 50 steel is comparable in tensile strength (not really relevant to a target) and superior in yield strength (a little relevant). It's about 10% lower in nominal Brinell hardness, which might concern me a little bit, but not a whole lot. It might worry me a bit more if I were shooting very hot .357 Magnum or some other high-energy revolver round.
For 9mm? I would shoot it in good health. Check it for excess deformation at the end of a given range session, and throw it out if the dents are getting deep.