From what I understand, there are three classes of what I would call Assault Rifles. The first is called Scout Rifle, and fills the role of a DMR. It is single shot, accurate and powerful. Then comes the Pulse Rifle, filling the role of a BR. It fires bursts that make it a good middle ground between the Scout Rifle and Auto Rifle. The Auto Rifle is the standard AR, fully automatic. Or at least, that's what I've gathered from my encounters with Destiny info pages. I think this is all right, but someone can feel free to jump in and correct me if I've made a mistake!