To curb the use of U.S.-made civilian guns in crimes and human rights abuses abroad, the Biden administration will require exporters to better vet their customers and tighten sales to 36 countries deemed “high-risk” for illicit diversion of semiautomatic firearms, according to people briefed on the plan. The new regulations—the most sweeping in decades—also cut the length of export licenses from four years to one and give the State Department greater authority to block sales.