http://www.kearms.com/resize/Shared/Images/Product/Billet-80-KE-15-Lower-Raw/80-ar15lower-raw-1.jpg?lr=t&bw=620&bh=500
That's a pic of an 80% lower. Essentially, it's not classified as a firearm because it's missing holes for essential components; mainly the fire control group. You have to cut all of that yourself.
https://www.lesbaer.com/images/BROACHEDLOW.jpg
That's a pic of a 0% lower. Basically nothing is done on it except for the magwell. Everything else needs to be done by you.
They are pretty much are just hunks of metal.
As for the NFA stuff in free states they are legal but require a tax stamp from the ATF. You can build an automatic gun but since it'd be an NFA item you'd need to apply for a stamp.
Also, I keep using the term free state because there are several states in the US that are only free on paper but in reality are not. NY, CA, CT, MD, NJ, MA, and a few others are not free states. They have some of the most ridiculous and stupid gun laws in the country.