Oh, that one is garbage. Horrible language. If I put a lighter trigger pull on my Glock, that's illegal.HR 3999: H.R.3999 - 115th Congress (2017-2018): To amend title 18, United States Code, to prohibit the manufacture, possession, or transfer of any part or combination of parts that is designed and functions to increase the rate of fire of a semiautomatic rifle but does not convert the semiautomatic rifle into a machinegun, and for other purposes.
A bit of a video about it, and some concerns here: House Bill Introduced To Ban Devices That Increase Rate Of Fire!
Fuck that.