I would check your sources on that. As one example, there are plenty of registered legal “Lightning Links” in the US. They are drop-in fully automatic on many (otherwise) unmodified AR-15 variants. With small exception, they shoot and cycle just fine. They’re literally just one small piece of steel.
One could make [1] their own in 2020 illegally with the hand tools you find at an Ace Hardware Store.
[1] - http://www.thehomegunsmith.com/pdf/fast_bunny.pdf