Legal principle from District of Columbia v. Heller (2008) holding that the Second Amendment protects arms "in common use" for lawful purposes like self-defense. Under this doctrine, weapons that millions of Americans own for lawful purposes can't be categorized as "dangerous and unusual" and therefore can't be banned outright. The Supreme Court's 2022 Bruen decision reinforced this framework by requiring governments to justify firearm regulations through historical tradition.