What makes assault rifles illegal?

Answer 1

The short answer is that they allow criminals to possess more firepower than police. A full-auto assault rifle can put remarkable amounts of bullets downrange, meaning that even a modest marksman can shoot his way out of a rough situation. The ammunition used by assault rifles also tends to be of a higher velocity; AK-47 and M16 rounds travel at about 3,000 fps. This allows them to penetrate some obstacles that average bullets could not, such as car doors and ballistic vests.

However, assault rifles that are not automatic use these same rounds, and are legal. So the short answer is that they fire too rapidly for the tastes of lawmakers.

Answer 2

Because a criminal can take an assault weapon and shoot, wound, and probably kill a great many people and it will take a lot of firepower to bring that person down. The law abiding citizen does not need an assault weapon, only the military and police need an assault weapon. The law abiding citizen can defend themselves with a single shot weapon, not an automatic weapon.