To make a long story short, an 80 lower is a items that have been machined to look like a firearm, but aren't yet. This is a marketing term that has been adopted by the firearms industry and is not recognized by BATFE (Bureau of Alcohol, Tobacco, Firearms and Explosives. BATFE only determines whether an item is a firearm or not. The term 80 lower or 80% Lower has grown from being just a term covering 80% AR-15 Lowers to many types of firearms as the DIY firearms industry developed around a single product.
Are 80 Lowers legal?
Chances are if you are purchasing an 80 lower from a reputable company they are legal. At one point, people were passing off firearms as 80 lowers and claiming they weren't firearms resulting in ATF raids. This isn't the case anymore, most manufacturers have received a determination form BATFE regarding their products. BATFE makes this extremely easy by submitting a sample to the Firearms Technology division.
History of 80 Lowers
80% Lowers started off as a home hobbyist DIY project for only the most daring of enthusiasts. When these started there were a few companies who would do runs of 25 or so of these lowers and sold them to their friends. Being aware of the laws and proper manufacturing techniques were the only way these projects were completed. Many of these enthusiasts would build from demilled parts and make extravagant builds out of what would seem like scrap parts to the average joe. With the advent of the internet and the creation of a commercial market, the industry grew rapidly. Now you can purchase many different types of 80 lowers.
Don't I need an FFL to make a firearm?
Contrary to popular belief, you don't need a Federal Firearms License (FFL) to manufacture firearms for your own personal use. This doesn't mean you can manufacture at home with the intent to sell or that you can make machine guns. If your intent is for your own personal use, you absolutely can make firearms at home. It's a great hobby and teaches about many things in life.
What about a serial number?
In most states, you aren't required to serialize a firearm that you manufacture at home. Don't take that as legal advice, seek out a knowledgeable lawyer in your area if you have any questions. Some states like New Jersey, New York and California are notoriously anti-gun and have made the waters muddy. Consult a lawyer in your area if there is any question of legality.
How hard is it to finish one?
This question brings us to our next topic, what do you mean by one? Who does one? No, seriously there are a few types of 80 Lowers now, so it depends. Reality is people in life are all created different. For some, this is a walk in the park, for others this may be a long expensive journey (me).
Completing an AR-15 80% Lower Receiver
Luckily in the interest of making a buck us red blooded americans have made it pretty easy for most people. If you have access to a palm router you can get this done if you're willing to spend money on a jig. After that, it's legos for adults, pick your parts slap it together, make sure everything fits and feels the way you want.
80% 1911 Frames
Probably the most difficult of the 80% lowers to convert to a working firearm is the 1911. Not because of any ridiculous milling requirements but primarily because all of the rest of the fitment after you've completed the frame. The 1911 is an old school hand fit firearm and it shows when you're doing an 80 1911 frame.
80% 308 Lowers
This is almost as easy as an 80% AR-15 lower. The milling is straight forward but sometimes the part selection after the fact is a bit confusing. You have to make sure you have the right parts picked out for your build. After you've picked everything out, you may have to revise your plan due to there not being a mil-spec and universal parts compatibility between manufacturers.
80% 10/22 Receivers
One of the most straight forward and technically challenging builds is the 10/22 receiver. What a fun project and what a way to get kids into building firearms. We need our next John Moses Browning and what a better way to start him or her out on a 10/22 build. Like the AR-15 80 lowers, these are well hashed out with parts but the actual manufacturing is slightly more difficult.