A firearm has one serial number. Federal law has required a single unique serial number on every gun manufactured or imported into the United States since the Gun Control Act of 1968, and it’s stamped or engraved on the frame or receiver. If you’re wondering how many serial numbers are on a gun and seeing different numbers on the slide, barrel, or other parts, those are not serial numbers. They serve other purposes.
Where the One Serial Number Is Located
The serial number is always on the frame or receiver because federal law designates that component as the legally regulated firearm.1Office of the Law Revision Counsel. 18 USC 923 – Licensing Exact placement varies by manufacturer and model.
On semi-automatic handguns, common spots include the frame near the trigger guard, above the grip, or on the underside beneath the barrel. Revolvers usually carry it on the frame below or in front of the cylinder, inside the crane, or on the butt of the grip. On rifles and shotguns, look at the receiver near the ejection port, behind the loading port, or under the top lever. Older designs sometimes tuck it behind the hammer or under the action lever.
If you can’t find it, check the owner’s manual or the manufacturer’s website. Most publish diagrams showing exactly where the serial number sits on each model.
Other Numbers That Are Not Serial Numbers
Most firearms carry several markings beyond the serial number, which is where the confusion starts. A barrel might show a part number, a batch code, or a caliber marking. The slide on a semi-automatic pistol often carries its own number stamped during manufacturing for quality control and parts matching. None of those are the gun’s serial number.
Proof marks are another common sight. These small stamps certify that the firearm passed pressure testing and met safety standards. European firearms in particular carry several proof marks from the country of manufacture. Importer markings appear on foreign-made guns sold in the U.S., because federal regulations require the importer’s name, city, and state to be engraved alongside the serial number.
When numbers appear on multiple components, the one on the frame or receiver is always the serial number of record. A different number on a slide or barrel usually means a replacement part was installed at some point. Licensed dealers who encounter mismatched numbers record the frame or receiver number and note the discrepancy in their records.
AR-15s and Other Split-Receiver Designs
Some firearms genuinely appear to have two serial numbers, and the AR-15 platform is the most familiar example. Its housing is split into upper and lower halves, and only one of them is legally the receiver.
Under the ATF’s 2022 final rule, the part requiring the serial number is the outermost housing designed to hold the primary fire control component. On most AR-pattern rifles, that’s the lower receiver. If a design uses more than one subpart to house the primary component, such as left and right halves of a clamshell receiver, each half must carry the same serial number.2eRegulations. 27 CFR 478.92 The rule also grandfathers earlier classifications, so firearms previously classified under the old system can keep their existing markings.3Bureau of Alcohol, Tobacco, Firearms and Explosives. Summary of Final Rule 2021R-05F
Guns That May Have No Serial Number
Not every firearm has one, and the absence isn’t automatically a red flag.
Antique firearms, defined under federal law as any firearm manufactured in or before 1898, fall outside the Gun Control Act’s requirements entirely.4Office of the Law Revision Counsel. 18 USC 921 – Definitions Many predate serial numbers as a standard practice. Muzzle-loading rifles, shotguns, and pistols designed to use black powder and incapable of firing fixed ammunition meet the same antique definition.
Guns manufactured between 1899 and October 22, 1968, sit in a gray area. The Gun Control Act took effect on that date, so anything made before it became law was not required to carry a serial number when it left the factory. Some manufacturers serialized voluntarily during this period. Others didn’t.
Privately made firearms, sometimes called ghost guns, are the other category. Federal law has long allowed individuals to make firearms for personal use without serializing them, though a 2022 ATF rule now requires licensed dealers to mark any privately made firearm with a serial number before transferring it to anyone other than the original owner.5Bureau of Alcohol, Tobacco, Firearms and Explosives. Definition of “Frame or Receiver” and Identification of Firearms
Missing or Altered Serial Numbers
If a firearm’s serial number has been scratched off, filed down, or otherwise altered, possession itself becomes a federal crime, provided the gun has at some point moved through interstate or foreign commerce.6Office of the Law Revision Counsel. 18 USC 922 – Unlawful Acts Because nearly every commercially manufactured firearm crosses state lines somewhere in its supply chain, that provision reaches broadly.
The penalty runs up to five years in federal prison, a fine, or both.7Office of the Law Revision Counsel. 18 USC 924 – Penalties You don’t need to have removed the number yourself to be prosecuted. Licensed dealers are prohibited from accepting a firearm with a defaced or missing serial number for sale, pawn, or transfer, which means a gun in that condition generally cannot be legally sold and loses its market value.
If you’re buying a used firearm, locate the serial number on the frame or receiver before the transaction and confirm it’s intact and readable. That’s the number that matters. Anything else stamped on the gun is something else.