Start at you local gun shop. Go with forged upper and lower receivers. 16.5 inch barrel.
Yep, although I’ve programmed and made both, the forged is actually superior over the billet except for the foo-foo additions.
However that being said, having the expanded flare on the magazine well Monty offers on his billet lowers for faster changes is a damn nice option to have also.
Is anyone selling cast receivers, these days?