rifles showing their configurations with different upper receivers. The lower receiver is visible at the bottom For the purposes of
United States law, the receiver or frame is legally the firearm, and as such it is the controlled part. The definition of which assembly is the legal receiver varies from firearm to firearm, under US law. Generally, the law requires licensed manufacturers and importers to mark the designated receiver with a
serial number, the manufacturer or importer, the model and caliber. Makers of receivers are restricted by
International Traffic in Arms Regulations. Thus, in the case of a firearm that has multiple receiver parts, such as the
AR-15, which has an upper and a lower receiver, the legally controlled part is the one that is serialized. although it is functionally a
chassis that also houses the separate
trigger group. In the
FN-FAL rifle, it is the upper assembly that is serialized and legally considered the receiver. This has led to prosecutors dropping charges against illegal manufacturing of AR-type firearms to avoid court precedents establishing that neither the upper nor the lower receiver individually contain all the components to be legally classified as a firearm.
Unfinished receivers "Unfinished receivers", also called "80 percent receivers", "80% lowers", or "blanks", are partially completed receivers with no serial numbers. Purchasers must perform their own finishing work in order to make the receiver usable. The finishing of receivers for sale or distribution by unlicensed persons is against US law. Because an unfinished 80% receiver is not a firearm, purchasers do not need to pass a background check. During the
Biden Administration, the
ATF imposed regulations on the sale and marketing of unfinished receivers and kits containing them by revising the legal definition of receivers to include "a partially complete, disassembled, or nonfunctional frame or receiver, including a frame or receiver parts kit, that is designed to or may readily be completed, assembled, restored, or otherwise converted to function as a frame or receiver". (The term "receiver" was also redefined as referring to non-handgun firearms, while "frame" was redefined as referring to handguns exclusively.) The new definitions went into effect on August 24, 2022. In the case
VanDerStok v. Garland, filed on June 30, 2023, a federal judge in the
United States District Court for the Northern District of Texas ruled that the new ATF regulations exceeded the agency's statutory authority, and struck them down. However, on August 8, 2023, the Texas court's nationwide
vacatur was temporarily placed on hold by the
Supreme Court of the United States, leaving the new ATF regulations on unfinished receivers in place. The case was appealed to the
United States Court of Appeals for the Fifth Circuit in November of 2023, with a three-judge panel upholding the District Court's ruling that the ATF had exceeded their statutory authority, striking the rule down; although the Supreme Court once again issued an emergency order allowing the ATF to continue enforcing the regulation while the litigation proceeded. On April 22, 2024 the case of
Bondi v. VanDerStok was accepted on appeal to the Supreme Court of the United States, and on March 26, 2025 in a 7-2 decision, the court upheld the Biden-era regulation, holding that "weapons parts kits are firearms within the scope of the
Gun Control Act" and affirmed the government’s authority to regulate them as such. This vacated the Fifth Circuit's ruling, allowing the ATF rule to remain in place. However, the Court's ruling did not outlaw all unfinished frames and receivers that are unserialized, it only applies to "certain partially complete frames or receivers sold in weapon part kits, which include everything needed to quickly assemble a working handgun", therefore the ruling primarily applies to 80% pistol frames bundled in kits. Unfinished
AR lowers are still legal under the Court's ruling, so long as they are sold individually and not as part of a kit with
jigs and instructions, and 76% pistol frames are still legal. As of March, 2026 unfinished AR lowers and 76 percent pistol frames are still being legally sold by companies like 80 Percent Arms and 80-Lowers.
State Bans A number of states have passed laws making "ghost guns" illegal by banning the sale of unfinished firearm frames and receivers, and the possession of unserialized firearms. As of March 26, 2026 unfinished frames and receivers, aka "80 percent lowers", and unserialized "ghost guns", are illegal in the following United States jurisdictions: •
California •
Colorado •
Connecticut •
Delaware •
District of Columbia •
Hawaii •
Illinois •
Maine •
Maryland •
Massachusetts •
Nevada •
New Jersey •
New York •
Oregon •
Puerto Rico •
Rhode Island •
Vermont •
Washington •
Philadelphia, PA ==3D printed receivers==