Also, the smart legislators are being a little careful. Folks are much more litigious now, and there is case law on individual ownership.
Remember, Miller said that the NFA was constitutional because the SBS in question wasn't useful in the militia. Specifically they said:
In the absence of any evidence tending to show that possession or use of a "shotgun having a barrel of less than eighteen inches in length" at this time has some reasonable relationship to the preservation or efficiency of a well regulated militia, we cannot say that the Second Amendment guarantees the right to keep and bear such an instrument. Certainly it is not within judicial notice that this weapon is any part of the ordinary military equipment, or that its use could contribute to the common defense.
The implication being that if it HAD been of use in the militia for common defense, it may have been protected. (Note I said may)
I think smart dems are as leery of bring an AR before the court while an M4 is THE weapon in use for common defense.
They are as leery of rolling the dice on SCOTUS as we were before Heller.
So lots of smoke and fire. Background check. Mag limits. Maybe "pistol" braces. But they (the smart ones) don't want to give us a hill to live or die on. (Metaphorically)
I also suspect they know they can expect ban compliance numbers lower than Australia and New Zealand's, which brings up the "now what" question.
The more ideological dems, or the dumber ones, haven't made those calculations, but the career Dems, I think, have.