NEW YORK POLICE WILL START USING DEADLIER BULLETS - NYTimes.com
"According to statistics released by the department, 15 innocent bystanders were struck by police officers using full-metal-jacket bullets during 1995 and 1996, the police said. Eight were hit directly, five were hit by bullets that had passed through other people and two were hit by bullets that had passed through objects.
In that same period, officers in the Transit Bureau, who already used the hollow points, struck six bystanders. Four of them were hit directly, one was hit by a bullet that ricocheted and another was hit by a bullet that passed through an object.
In that same period, 44 police officers were struck by police gunfire using the old ammunition: 21 were hit directly, 2 were struck by bullets that ricocheted and 17 were struck by bullets that passed though other people. Of the four officers struck by hollow-point bullets, three were hit directly and one was hit by a bullet that passed through another person."
So, over those two years, 37% of those people unintentionally shot by the NYPD with FMJ rounds were hit after the bullet had already passed through another person. Of those unintentionally shot with HP, 10%. These aren't large enough samples to be definitive, but it certainly indicates that overpenetration is a legitimate concern.