Cornell University law professor Michael C. Dorf explains why, if Congress wants to ban or further regulated the sale of “bump stocks,” it should act quickly or risk missing the window in which regulation is possible. Dorf points out that the test the Supreme Court uses for whether weapons count as “arms” protected by the Second Amendment is whether they are in “common use,” not whether they are “dangerous and unusual weapons.” Dorf argues that so long as bump stocks remain legal, people can accumulate them, and if enough people do that before they are banned, there could be so many in circulation as to qualify as in common use, thereby falling within the scope of Second Amendment protection.