Just taking a quick look, here is the number of officers killed in 2003&2004 with any kind of rifle: 23. In 2005&2006: 11.
http://www.fbi.gov/ucr/killed/2006/table27.html I smell ample amounts of BS here and I think you found the motherlode - they omit the numbers during the ban intentionally to make a non-story. It's hard to duplicate their numbers without finding out what carefully chosen criteria they selected to define "assault rifle," but aggregate rile statistics tell the story.