A ghost gun is a term for a (typically) homemade firearm that lacks commercial serial numbers. The term is used mostly in the United States by gun control advocates, gun rights advocates, law enforcement, and some in the firearm industry. Because home-manufacture of firearms for personal use is not considered to fall under the federal government’s authority to regulate interstate (as opposed to intrastate) commerce, individuals making their own firearms are not subject to federal or state commercial background check regulations. Persons otherwise prohibited from owning firearms are still legally barred from the manufacture, transfer, or possession of firearms or ammunition, regardless of the method of manufacture or acquisition.
Under U.S. federal law, the creation and possession of firearms for non-commercial purposes has always been legal. In contrast, firearms for sale or distribution require a federal license for their manufacture, and must bear unique serial numbers.
A firearm receiver, which under US law is most often the part legally considered to be a "firearm," can be completed from raw material, an "unfinished receiver," or a so-called "80% receiver," the last of these being a non-legal term the Bureau of Alcohol Tobacco and Firearms, or ATF, does not recognize. While some states have passed laws restricting the creation of ghost guns, in most states unfinished receivers are sold without the requirement of federal or state background checks. Finishing work on these receivers may be performed with machine tools, the common drill press, or hand-held tools like the Dremel. Companies sell kits that include drill bits, stencils, and jigs to aid the process, but proficiency with tools is usually required.
It has always been possible to make firearms from raw materials, and more recently it has become popular to produce receivers from plastic with a 3D printer, though the variety of materials and methods used to create these produce receivers of greatly varying quality. The most popular machine tool for completing 80% receivers is a CNC mill by Defense Distributed named Ghost Gunner, which is optimized for completing aluminum and steel unfinished receivers.
AR-15-style firearms are often made as ghost guns. AR-15s are modular firearms, and maker's marks are usually applied to the lower receiver, which houses the trigger group. A person with an AR-15 lower receiver can assemble a complete weapon using widely available components, such as barrels, stocks, magazines, and upper receivers. Pistols and AK-47-style semi-automatic rifles are also popularly made as ghost guns.
Overseas production centers of unmarked firearms include China, the Khyber Pass area of Pakistan and the Philippines; the Philippines are especially known for the production of .45 caliber semi-automatic pistols.
In recent years, politicians and gun control advocates have claimed tracing unserialized firearms used in crimes may be harder than tracing their commercial variants. This argument is made when assessing ATF's ability to use the serial numbers of firearms associated with crimes to perform "trace requests" through The National Tracing Center (NTC), a division of the bureau that provides investigative leads for local, state, federal and foreign law enforcement agencies. However, ATF NTC uses other technologies and forensic expertise when serial numbers are not present, or have been altered or obliterated from firearms, and local detectives, prosecutors and U.S. Attorneys have many other resources and investigative powers aside from NTC when working gun crimes, including the NIBIN. Additionally, as was seen in Operation Fast and Furious the ATF was unable to trace their own guns that had serial numbers.
While there are no reliable statistics on how many privately made firearms are being recovered in crimes, since the issue rose to prominence in California, ATF has documented recoveries of ghost guns in 38 States plus DC, Puerto Rico, and the Virgin Islands. ATF noted an increasing number of ghost gun seizures every year since 2016, and over 1,600 of these firearms have been entered into NIBIN.
Gun rights and other American political activists support the private production of firearms, claiming the practice as a Constitutional right and a way to maintain the privacy of gun owners. Individuals have organized "build parties" where equipment and expertise are shared to help create ghost guns. Advocates say that ghost guns are rarely used in crime despite widespread ownership. Gun rights advocates and law enforcement assert that due to the cost and effort required to make ghost guns, criminals prefer to steal the guns used in crimes, a fact borne out by DOJ statistics. Between 2012 and 2017, ATF estimated over 1.8 million firearms were stolen from individual gun owners, vehicles and residences, and another 40,000 were stolen from FFL's, numbers that vastly dwarf those of ghost guns linked to crimes.
Unserialized firearms have been used in at least two shooting sprees in California, most notably one at Rancho Tehama Reserve in 2017 by a man who was barred from possessing guns. The shooter also carried and primarily used three firearms he had illegally acquired that were not ghost guns.  Additionally, firearms with scraped off serial numbers have been used in numerous crimes, however, an altered gun is not the same as a homemade firearm.
Congress passed the Gun Control Act of 1968 or the GCA, to expand interstate commerce controls over common firearms like pistols, revolvers, shotguns and rifles. The GCA requires those who are “engaged in the business” of manufacturing or dealing in firearms to be licensed by ATF. Federal firearms licensees are required to mark their firearms’ serial numbers and keep records of their transactions. The GCA also prohibits certain categories of persons, like convicted felons, domestic abusers, illegal drug users and others, from possessing firearms.
To help enforce these prohibitions, Congress passed the Brady Act in 1993, creating the National Instant Criminal Background Check System, or NICS, and requiring FFLs to submit potential firearms purchaser information to NICS before transferring firearms.
While Congress passed the GCA to help law enforcement fight gun crime, it expressly added that the Act was not to place any undue burden on law abiding citizens who use or make firearms for lawful, private purposes.
In 2014, California attempted to enact a law to require serial numbers on unfinished receivers and all other firearms, including antique guns, but it was vetoed by the governor. However, in 2016, it passed a measure requiring anyone planning to build a homemade firearm to obtain a serial number from the state (de facto registration) and pass a background check. From July 1, 2024, "firearm precursor parts" may only be sold through a licensed dealer.
Since October 1, 2019, all manufactured guns must have a serial number obtained from the Department of Emergency Services and Public Protection engraved. Any Plastic gun that "after removal of grips, stocks and magazines, is not ...detectible" by metal detectors is banned under Connecticut law.
S2465, enacted in November 2018, prohibits the manufacture and sale of guns or parts that are or can become an untraceable firearm. Multiple arrests were made within months of this law going into effect. Attorney General Gurbir Grewal aggressively prosecuted infractions of this law. New Jersey filed a lawsuit against U.S. Patriot Armory, a company that allegedly sold AR-15 build kits to NJ residents. In July 2019, S3897 was enacted, which criminalizes transferring or possessing unserialized firearms.
In 2015, during the state of New York's first prosecution for sale of ghost guns, Attorney General Eric Schneiderman said that it was "easy" for "criminals to make completely untraceable, military-grade firearms." In 2019, New York passed a law to prohibit the making, selling, transporting or possessing 3-D-printed guns or other undetectable firearms.
Attorney General Josh Shapiro issued a legal opinion in December 2019 that 80% lower receivers are considered firearms. A legal challenge ensued and in January 2020 Commonwealth Court issued a preliminary injunction blocking AG Shapiro's opinion.
On July 1, 2020, Representatives Jamie Raskin (MD-08) and David Cicilline (RI-01) introduced House Resolution 7468, aiming to reduce gun violence through banning the manufacture of "ghost guns"—firearms that require no background check, have no serial number, and are essentially impossible to trace. As of September 22, 2020, the most recent action taken on the bill was on July 1, when it was referred to the House Committee on the Judiciary.
As of April 2020, there are at least two bills that aim to control the distribution of firearm kits as well as 3D printed weapons in the Commonwealth: Bill H.3843, "An Act relative to ghost guns", presented by Marjorie C. Decker of 25th Middlesex district, and Bill S.2649, "An Act relative to 3D printed weapons and ghost guns", presented by Michael J. Barrett of 3rd Middlesex district. Both bills have been deferred to the Committee of Ways and Means in the Senate and House, respectively.
On February 7, 2019, Illinois House Rep. Kathleen Willis filed HB2253, entitled the Undetectable and Untraceable Firearms Act, with the Clerk of the House was the Bill was announced to the House.  It was then referred to the House Rules Committee for assignment to a substantive committee, and to be formally heard by lawmakers and the public.  The Untraceable Firearms Act, for short, proposes to amend the Firearm Owners Identification Card Act primarily by prohibiting the possession, manufacturing, and distribution of "unfinished frames or receivers" without having a FOID (Firearm Owners Identification Card) in his or her possession, among other requirements.  HB2253 also proposes to include these "Ghost Guns" as prohibited weapons in certain areas, including public buildings. Violations of HB2253 would result in the commission of a Class 2 felony, punishable by 3 to 7 years in the Illinois Department of Corrections and fines up to $25,000. 
The Bill has garnered both support and criticism among lawmakers. In the Bill's introduction, Rep. Willis stated, "I'm not calling for a ban on them, I'm just saying that you need to have the same background checks as you would if you were going to purchase a regular gun..."  On the other hand, the Federal Firearms Licensees of Illinois have voiced 2nd Amendment concerns on behalf of gun sellers: "[Rep. Willis is] trying to make it illegal for the home hobbyist to own or possess firearms they've made. They're going after an industry and a hobby and lawful gun owners."