This is what a M16 Cut lower looks like on the inside.
This company sells them and all they need is the Third Hole
https://stdgun.com/standard-mfg-stripped-lower-receiver/
The shelf would need to be opened up but yes it would make an awesome full auto.
And doing the shelf is easy peazy because I have a Milling Machine.
The milling of the shelf is not illegal but drilling the third hole is.