Looks like Minnesota Democrats are on board. Here's some language from the gun control bill they've proposed.
10.20 Sec. 7. PERSONS POSSESSING ASSAULT WEAPONS ON EFFECTIVE DATE ACT; REQUIRED ACTIONS.
Any person who, on February 1, 2013, legally owns or is in possession of an assault weapon has until September 1, 2013, to do any of the following without being subject to
prosecution under Minnesota Statutes, section 624.7133:
(1) remove the weapon from the state;
(2) surrender the weapon to a law enforcement agency for destruction;
(3) render the weapon permanently inoperable; or
(4) if eligible, register the weapon as provided in Minnesota Statutes, section 624.7133, subdivision 5.
EFFECTIVE DATE.This section is effective the day following final enactment.