Usually if no forward assist, the whole lower receiver is "smooth" and has no dust door, just an open hole the brass flies out of. To see what manufaCTURERS are doing inside the guns and how they compare, google "So you want to buy an AR15 huh?" and read that thread. A LOT of info about the guns that are available out there in this thread, materials, how tested, feed ramps, M4 feed ramps and which type uppers they best work in, which companies are doing it that way, which are not, and so on.
Russellc