When Killer is in his car staking out the old man's apartment, you hear the sound of him cocking his semi auto pistol, which takes two hands. As this takes place, you see him put the gun into his holster with one hand.
When Driver kills his first victim in a point blank range, the cubicle wall behind the victim is splattered with blood. Later when Cop is watching the surveillance footage of Driver and the victim, the blood splatter on the wall is smaller.
At the end of the movie when Cop is lying on the ground, we see (from Cop's POV) Driver fire his gun into his face. In subsequent shots his face is undamaged (and in fact no part of his body appears to have sustained an additional gunshot wound).