The Brenneke slug may not penetrate but will at least incapcitate and possibly kill the BG/victim. The flex in the vest will extend some inches - 4" ???- what ever tissue behind that will get damaged. Read a story about a SWAT officer hit by a Brenneke Slug and died as a bone (rib or strenum or collar bone) was broken and penetrated his heart. The Box-O-Truth tested a soft Foster style slug and still left quite an impression behind the vest. The Brenneke hard cast slug would not flatten out and would "penetrate" - i.e. push into the victim much further. For sure the bg/victim would be incapacitated even if they lived which they probably would.