"Congressional interest in the legal framework for regulating 'bump stock' devices abounded after authorities discovered that the perpetrator of the October 2017 mass shooting in Las Vegas, Nevada, had attached to his semiautomatic firearms an accessory that allowed his rifles to effectively mimic the firing capabilities of a fully automatic weapon. (These firearm accessories are commonly called 'bump-fire,' 'slide-fire,' or 'bump-stock' devices, and more information on how they function can be found in this CRS Insight by William J. Krouse.) One pertinent question involves the degree to which the Department of Justice's Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF) may regulate, and potentially proscribe, the sale and possession of such devices under existing law. The answer to this question largely depends on whether a bump-stock device reasonably can be construed as falling under the federal statutory definition of a 'machinegun.'"
CRS Legal Sidebar, LSB10103
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html