The second you get specific, the concept stops being politically useful. Most of the politicians discussing this aren't even clear on "automatic" and "semi-automatic."

As I understand it, the original "assault weapon" ban was selected mostly by appearance of the weapon by people who had no idea what they were looking at. I didn't find the Federal ban flowchart, but there's a California version here.

Automatic weapons are already very tightly restricted, and figure in none of these mass killings. In fact, legally-registered ones have accounted for two deaths since 1936. Obviously, illegal ones are another story. The process of owning one legally (it is possible) is described here.