On 14 December 2012, a young man entered Sandy Hook Elementary School in Newtown, Connecticut, USA. He brought with him several guns, including two handguns and an AR15-type rifle. He had killed his mother in her home that morning. At the school, he killed an additional 26 people, including 20 children. He then took his own life.

Following the Newtown shooting, numerous bills were introduced in the USA, at both the state and federal levels, to better regulate access to firearms. At the federal level, the primary bill was the so-called Manchin-Toomey amendment, named for its chief architects in the US Senate. That bill would have expanded background checks of gun buyers to include most sales at gun shows and over the internet. Current US federal law only requires a background check prior to a gun sale by a licensed gun dealer. The Manchin-Toomey amendment garnered substantial support, but ultimately received 54 votes in the Senate, falling 6 votes short of the number needed to overcome a procedural hurdle.

At the state level, the news was considerably better. Fifteen states and the District of Columbia—jurisdictions in which approximately 44% of the US population lives—enacted new laws, making it harder for …