Bump Stock

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A bump stock makes semiautomatic RIFLE (not handgun) shoot faster (than without the bump stock). Still a bump stock will not make a rifle automatic (which shoots even faster than a rifle equipped with a bump stock), shooting continuously by pulling the trigger just once.

(1) There is a phrase: lock, stock, and barrel
https://en.wikipedia.org/wiki/Lock,_stock,_and_barrel
, which teaches you about a stock, as part of a gun.

(2) What Is a Bump Stock and How Does It Work? New York Times, Mar 28, 2021
("A 'bump stock' replaces a rifle's standard stock, which is the part held against the shoulder. It frees the weapon to slide back and forth rapidly, harnessing the energy from the kickback shooters feel when the weapon fires. The stock 'bumps' back and forth between the shooter's shoulder and trigger finger, causing the rifle to rapidly fire again and again")

(3) bump stock
https://en.wikipedia.org/wiki/Bump_stock
, whose top figure shows a bump stock (dull black, in the left) attached to a semiautomatic rifle (shiny black, to the right) the two are detachable, and sold separately. Figure 2 in this Wiki page is an animation that shows how a bump stock works, with a caption: "A bump stock causes the trigger (red) to be actuated when the receiver moves forward, being reset each round by receiver recoil. This allows semi-automatic firearms to somewhat mimic fully automatic weapons."

actuate (v; etymology: "borrowed from Medieval Latin āctuātus, past participle of [verb] āctuāre to actualize, verbal derivative of Latin [noun masculine] āctus ACT entry 1"): "to put into mechanical action or motion"
https://www.merriam-webster.com/dictionary/actuate

receiver (firearms)
https://en.wikipedia.org/wiki/Receiver_(firearms)  
(also called frame)

Figure 3 of this Wiki page shows law of various states regarding legality of bump stock in each state. However, a 2019 regulation by Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF; within US Department of Justice) that classified a bump stock as tool for automatic weapon banned bump stock at the federal level, thereby superceding state laws. The regulation is published in Code of Federal Regulations (CFR)
https://en.wikipedia.org/wiki/Code_of_Federal_Regulations
(section 1 Background)

(4)
(a) Under Separation of power, Congress makes laws. Executive branches interpret laws with CFR. If Congress disagrees with CFR, it may change laws, in part to make it more explicit, less unambiguous.
(b) Challenges to ATF's rulemaking in this regard has been repeatedly upheld. See Cargill v Garland (CA5 2021) 20 G.4th 1004, 1006 and n 2.
https://casetext.com/case/cargill-v-garland
(i) CA5 is a shorthand for United States Court of Appeals for the Fifth Circuit, based in New Orleans, Louisiana/
(ii) Cargill and Garland were the surnames of plaintiff and defendants, the latter (Merrick Garland) being sued in his official capacity as United States Attorney General. F stands for Federal Reporter (published by West Publishing division of Thomson Reuters Corp. The 4th signifies the fourth edition of Federal Reporters. The 20 and 1004 are volume and starting page numbers, respectively. The convention of in legal field place volume number in front of the publication, whereas in biology, volume number FOLLOWS publication (name). The 1006 is the page your attention is directed to, "and" wants you to read footnote 2 also (in case your attention is directed to footnote 2 only, but not the text in other part of page 1006, it would be written instead: 20 G.4th 1004, 1006, n 2.

The above footnote 2 mentioned Aposhian v Garland, Supreme Court No 21-159, which remains under consideration (whether Supreme Court should accept it for review.
https://www.supremecourt.gov/sea ... /public/21-159.html