The Supreme Court’s conservative supermajority carved a huge loophole into the federal prohibition against machine guns on Friday, striking down a bump stock ban first enacted in 2018 by the Trump administration. Its 6–3 decision allows civilians to convert AR-15–style rifles into automatic weapons that can fire at a rate of 400–800 rounds per minute. One might hope a ruling that stands to inflict so much carnage would, at least, be indisputably compelled by law. It is not. Far from it: To...