NOTE: I am not an attorney or in any way qualified to give legal advice and nothing here or elsewhere on this website should be construed as legal advice.
Is it legal to create a firearm from an 80% lower base? The ATF says yes, with the usual exceptions. If you are prohibited from owning a firearm you cannot make one either. You should check with your state and local laws as well to make sure they don't prohibit it. You cannot make a firearm that is otherwise prohibited in your locality. So if you live in an area that prohibits certain firearm configurations then any firearm you build must follow these laws. You also cannot make the firearm with the intention of selling it. Being able to make a firearm for your own use is very different from being a licensed manufacturer.
Here is what the ATF website says about making your own firearm (not specific to 80% builds).
"Q: Does the GCA prohibit anyone from making a handgun, shotgun or rifle?
With certain exceptions a firearm may be made by a non-licensee provided it is not for sale and the maker is not prohibited from possessing firearms. However, a person is prohibited from assembling a non-sporting semi-automatic rifle or non-sporting shotgun from imported parts. In addition, the making of an NFA firearm requires a tax payment and approval by ATF. An application to make a machine gun will not be approved unless documentation is submitted showing that the firearm is being made for a Federal or State agency.
[18 U.S.C. 922(o) and (r), 26 U.S.C. 5822, 27 CFR 478.39, 479.62 and 479.105]"
The section above about making a "non-sporting" firearm should not be an issue with almost any AR builds. While the standard configuration AR is "non-sporting" most parts for the AR are built in the USA. The "non-sporting" section (922r) is more an issue for other firearms with a foreign pedigree such as the AK style rifle which are usually made overseas. It should be noted that section 922r does not apply to handguns.
In case you don't know, "NFA" stands for National Firearms Act. This was a national act passed in 1934 and it basically covers machine guns, silencers (suppressors), short barrel rifles (SBR), short barrel shotguns (SBS), any other weapons (AOW) and more recently, destructive devices (DD). When building an AR-style rifle you want to make sure you keep the barrel length 16" or greater. Any barrel shorter than that would make it an SBR. Or if you are making an AR handgun do not put a forward grip on it as this would make it an AOW. You cannot make a machine gun either but I'd like to think that goes without saying.
Here is what ATF says about selling a firearm you have made:
"Q: Is it legal to assemble a firearm from commercially available parts kits that can be purchased via internet or shotgun news?
Individuals manufacturing sporting-type firearms for their own use need not hold Federal Firearms Licenses (FFLs). However, we suggest that the manufacturer at least identify the firearm with a serial number as a safeguard in the event that the firearm is lost or stolen. Also, the firearm should be identified as required in 27 CFR 478.92 if it is sold or otherwise lawfully transferred in the future."
Now that we've covered the why and some of the surrounding laws let's talk about what type of material to choose for your 80% lower project. Click here