Why? And where (if anywhere) would you draw a line?

United States v. Miller (as cited in District of Columbia v. Heller) found that the Second Amendment applied to guns “in common use at the time". Most, if not all, things banned by the assault weapons ban are in common use at this time. In fact, the 1994 law banned AR-15s by name, which are now one of the most popular rifles. That's pretty much the definition of "in common use at the time".