the maximum and minimum output voltage of op-amp
Best option will likely be to adjust the feedback resistor on your op amp so that you can accurately measure from 0-0.5A using voltage range 0.05-4.8 that the op amp is guaranteed capable of.

No amplifier that I'm aware of can operate at steady-state at or beyond it's "rails" (although there are several AC amps that can momentarily do that).