They are not firearms by Federal definition. They are not something one can jsut finish out with a dremel. A lot of folks buy them in hopes of making an "untraceable" AR. Most find 20% is still a bunch of work.
First, welcome to the forum. Stop by the intro thread and say hello.
Second, "Legal" means "complies with the law". Question is- WHICH law? We have 50 (or 54 according to some folks) states, plus FEDERAL law. And YOU, sirrah, did not specify which state.
Now, using my Moderator Superpowers, I detect a trace of a Jersey accent in your speech. Your first question will be is an AR-15 rifle- complete, whole and functioning- legal in Jersey.
I am not a lawyer- but the answer is "Maybe". Jersey has restrictions of what THEY call an Assault Rifle. To acquire an Assault Rifle in Jersey, you need a permit. It is my understanding they do not like to give those out. However, there is a link to Jersey gun laws at the bottom- click on it, and read carefully- and talk to some of the gun shop owners there in Jersey.
Now, as to the 80% lower- under both State and Federal law- it is NOT yet a firearm- it is a lump of metal. Lumps of metal are not regulated by gun laws. It IS legal to make your own gun for your own use (not for sale)- but again, Jersey has permit requirements. And you may find that final 20% of precision machining is harder than you think unless you are a tool and die maker with a lathe and a milling machine.