Depends on who does the 20%
Right. A Dremel or hand drill is not the most accurate way to go.
Exactly. I build mine 100% from both billet and forging, but I own a tool and die shop so I kind of cheat. Some of the kits that come with the jigs can make an acceptable one, to be honest you’re not really going to see any difference at all imho. All depends on the upper and the components you choose to finish it off. As long as the trigger holes and pocket are close enough the trigger sits correctly, and the mag catch is at the proper location with high quality magazines the rest of it is meaningless.
I always wondered why the ATF chose the lower to make 80% and not the upper. the upper is where the skill comes in to finish.