There is a German bullet manufacturer who has been playing with that petal design.
http://www.lima-wiederladetechnik.de...rgeschosse.jpg
and one of before and after bullet performance:
http://www.lima-wiederladetechnik.de...er-nachher.jpg
His Whisper page is in German but Babelfish etc works well enough on it.
http://www.lima-wiederladetechnik.de...er-Patrone.htm
Once again not cheap and also gets away from the "tumbling" effect of a mega-weight projectile. It is more using supersonic theory in a subsonic way.
BTW Schalldämpfers is Silencer.