The Massachusetts Attorney General issued an Enforcement Notice in 2016 to announce a new interpretation of a key phrase in the state's assault weapons ban. The Enforcement Notice increased sales of tagged assault rifles by 616% in the first 5 days, followed by a 9% decrease over the next three weeks. Sales of Handguns and Shotguns did not change significantly. Tagged assault rifle sales fell 28-30% in 2017 compared to previous years, suggesting that the Enforcement Notice reduced assault weapon sales but also that many banned weapons continued to be sold. Tagged assault rifles sold most in 2017 in zip codes with higher household incomes and proportions of white males. Overall, the results suggest that the firearm market reacts rapidly to policy changes and partially complies with firearm restrictions.