Closing the Bump-Stock Loophole Act This bill amends the Internal Revenue Code to allow the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) to regulate a reciprocating stock, or any other device which is designed to accelerate substantially the rate of fire of a semiautomatic weapon (i.e., bump stocks) in the same manner as a machine gun. Any person possessing such a device when this bill is enacted must register the device with the ATF within one year of the enactment of this bill, and the registration must be included in the National Firearms Registration and Transfer Record. The bill defines a "semiautomatic weapon" as any repeating weapon that: (1) utilizes a portion of the energy of a firing cartridge to extract the fired cartridge case and chamber the next round, and (2) requires a separate function of the trigger to fire each cartridge.