This is not legal advice, it's not even my wording.
I copied it from;
80% Lower Receiver Information | Tactical Machining
What is an 80% receiver?
An 80% Receiver is a partially completed piece of material that requires special tooling and skills to be completed and considered a firearm. Unlike a firearm receiver it is not required to be transferred through a Federal Firearms Licensed holder (FFL).
Legalities
A lot of questions have been raised about how our 80% AR-15 Lower Receivers can be considered non-firearms. We've received a determination letter from the ATF stating that the ATF does not view this as a firearm because to complete the receiver it requires specialized skills and tooling.
Do I need an FFL to make a firearm?
In a short answer, no. As long as you can legally own the firearm in the state, county and city you reside in you are allowed to manufacture a firearm for your own personal use. No serial number is required however it is suggested to engrave a serial number, model and manufacturer on your receiver in case it is lost or stolen.
All National Firearms Act (NFA) regulations apply to your firearm. You may create a short-barreled rifle out of your firearm via a BATFE (Bureau of Alcohol Tobacco Firearms and Explosive) Form 1 after BATFE approval.
Recommended Tools
Completing an 80% Lower Receiver can be accomplished using a variety of tooling and methods, the most common methods require using a manual mill or a drill press. Both manual mills and drill presses can be purchased off web sites like craigslist or ebay as well as from local tooling suppliers. Once you have the machine of your choice, end mills and drills will be required to complete the remaining operations.
Resources
There are many resources on the internet you can use to acquire help and information from the home build gunsmith community. Some of our recommended forums are the following: Tactical Machining Build it yourself forums. Our resource center includes information and advice to commonly asked questions as well as direct answers from us on any question you may have. Weapons Guild & Home Gunsmith - Weapons guild and home gun smith are two great communities of home builders who would be more than happy to help you in your 80% completion project.