Very bad advice, unless you are attacked by sheets of paper. A soft lead slug traveling anywhere from 650-700 fps are only going to make someone angry.
I just checked my logbook....looking through my records for my 2" J-frame. The 148 gr wadcutters I shot in the J-frame, which were light loads intended only for bullseye shooting, ran 701 FPS. That is pretty close to your stated velocity.
Am I correct in my understanding that the people you referenced (inner city whatevers) were using light weight target hand loads for self defense?