Traditionally, the semiautomatic rifle he had is not defined as an assault rifle. The media and politicians have changed public perception to attach "assault" based on the looks of the gun. Please watch this video for an explanation of rifle types and also a demonstration of how the same inner gun can be made to look more menacing with some quick changes : http://www.youtube.com/watch?v=yATeti5GmI8

I love disolved gas! It's fascinating stuff, though - traditions being inspected by modern science.