Mass. – Everytown filed this amicus brief in the United States Court of Appeals for the First Circuit in defense of Massachusetts’s law prohibiting assault weapons and large-capacity magazines. This brief details the long history of regulating especially dangerous weapons, including a century of restrictions on semiautomatic firearms capable of firing a large number of rounds without reloading. It also demonstrates the circularity and illogic of the “common use” test argued for by plaintiffs, based upon nationwide sales and manufacturing figures, which cannot be reconciled with either the Supreme Court’s decision in Heller or common sense, and is inconsistent with core principles of federalism. And this brief further explains how plaintiffs have not met their burden of showing that Massachusetts’s law substantially restricts their ability to defend themselves with firearms.