Did I misquote the title?Go back and reread the OP.There was more to the question, but don't let that bother you.i think you answered your question one is automatic the other is not.The M4 is the main rifle used by the US military, and no one could question that it is an assault rifle. The M4 is capable of full automatic fire, and 3 round burst, and the AR15 is not. Of course, it would be illegal, but the AR15 can easily be converted to allow those types of fire. Other than that difference, what makes an M4 an assault weapon, and an AR15 not?Nobody doubts the M4 is an assault weapon. Are there any material differences between an M4 and an AR15?