That's a pretty nasty surprise for anyone who bought these products, which were repeatedly declared legal by the Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF) during the Bush and Obama administrations. It is especially unsettling because the law has not changed since the agency made that determination.
What happened instead is that the ATF, under orders from Donald Trump, reinterpreted the law to reach a diametrically opposed conclusion that is contrary to the plain meaning of the relevant statutes. Critics of presidential power grabs, regardless of how they feel about bump stocks or gun control more generally, should be troubled by this blatant usurpation of congressional authority.
Bump stocks are accessories that facilitate a rifle shooting technique in which recoil energy causes the gun to slide backward after a round is fired, resetting the trigger. By maintaining forward pressure on the rifle, the shooter bumps the trigger against his stationary finger, which fires another round, causing the rifle to slide backward again, and so on. Until October 1, 2017, the technique, which increases the rate of fire but reduces accuracy, was of interest mainly to gun enthusiasts, the companies that sold bump stocks, and the ATF officials who approved their sale. But after a gunman equipped with multiple rifles and bump stocks murdered 58 people in Las Vegas, Trump promised to ban the devices by administrative fiat.