It's generally a crime to possess a machinegun. If you affix a "bump stock" to a semiautomatic rifle, it functions essentially like a machinegun. So, do you now have a "machinegun"? ATF: yes. District court: yes. Sixth Circuit (en banc) (2021): We are evenly split and cannot decide. What do you want us to do? Sixth Circuit (three-judge panel, 2023): Um, since our full court can't decide this, and a bunch of other judges are all over the place, and the same question is before us yet again, let's just go with the rule of lenity. Not a machinegun. Concurrence: Lenity, schmenity, until Congress says otherwise, it's not a "machinegun." We don't get to expand criminal law.
The case is Hardin v. Bureau of Alcohol, Tobacco, Firearms & Explosives, No. 20-6380 (6th Cir. Apr. 25, 2023).