The law says that businesses manufacturing guns need to keep records and add a serial number to each gun. That number is usually stamped or molded into the gun. Guns have several parts many of which are made to be replaced after they are worn. Some guns have modular designs allowing for customization with different barrel lengths and different stocks for people with longer or shorter arms. It is the part called the lower receiver that gets the serial number.
Large manufacturers have automation and assembly lines to keep manufacturing costs low and to keep quality consistent. Expensive machinery is not required to make a gun. Except for the lower receivers, almost all of the other parts can be bought from mail-order catalogs or via the internet. So now the focus is on the "lower". Lowers begin their lives as hunks of raw metal and are machined into lower receivers. At some point between being raw metal and finished products, these pieces of metal become guns according to the legal definition used by the ATF. According to the ATF, when the piece of metal is 80% finished, it still isn't a gun legally. Therefore, 80% lowers are sold through the mail like all others gun parts along with plans for how to finish them. All of this is legal according to ATF unless you are prohibited person (felon, domestic abuser, dishonorably discharged...).
Typically a drill press and some hand and power tools would be needed to complete the finishing on the 80% lower. Drill presses go for $70 to $700 but I've seen them for $30 at a yard sale a few years ago.
Now for the news: Syracuse joins lawsuit against feds amid rise in ghost guns
https://www.wrvo.org/post/syracuse-joins-lawsuit-against-feds-amid-rise-ghost-guns
A ghost gun is a homemade firearm cobbled together from easy to get gun parts. Those parts are currently not regulated by the ATF, an agency of the federal government. So, the guns skirt firearm laws. Walsh has joined mayors from other cities including Chicago in a lawsuit that would force the ATF to change its policy on gun parts.
"We need the gun components to be considered firearms, and regulated the same way, so people cant easily order them online, get them mailed, and put together, which is like any other firearm that is regulated, Walsh said.
The group Everytown for Gun Safety, a gun violence prevention organization, is leading the lawsuit. Its research shows ghost guns are becoming a weapon of choice for people with felony convictions, gun traffickers and other people legally prohibited from owning guns.
A six-year-old boy was seriously injured in Syracuse late last year by gunfire from a ghost gun. The cities of San Jose, California and Columbia, South Carolina are also joining in the lawsuit.