How well do firearm markets comply with firearm restrictions? The Massachusetts Attorney General issued an Enforcement Notice in 2016 to announce a new interpretation of the key phrase “copies and duplicates” in the state’s assault weapons ban. The Enforcement Notice increased assault rifle sales by 1349 (+560%) within 5 days, followed by a reduction of 211 (−58%) over the next 3 weeks. Assault rifle sales were 64%–66% lower in 2017 than in comparable earlier periods, suggesting that the Enforcement Notice reduced assault weapon sales but also that many banned weapons continued to be sold.