Yes, you can buy an upper receiver in California. Federal law does not classify an AR-15 upper as a firearm, so there is no background check, no Form 4473, and no FFL transfer required. You can order one online and have it shipped to your home. The catch is everything that happens after the box arrives: the features on the upper, the barrel length, the lower you pair it with, and the magazine you load all decide whether you end up with a legal rifle or a felony.
Why No Background Check Is Required
Under ATF rules, the “firearm” on an AR-15 is the lower receiver, the part that houses the trigger mechanism and hammer.1eCFR. 27 CFR 478.12 – Definition of Frame or Receiver The upper holds the barrel, bolt carrier group, and charging handle, and it falls outside that definition. Because it is not a firearm, it does not need a serial number, and the sale does not go through a licensed dealer. That is true whether you walk into a store or buy online from another state.
Getting an Upper Shipped to California
You can have an upper shipped straight to your home address. In practice, two things make this harder in California than in most other states.
AB 1263 Shipping Rules Starting January 1, 2026
Beginning in 2026, retailers shipping firearm barrels, firearm accessories, and firearm manufacturing equipment to California addresses must follow new procedures under amended Civil Code Section 3273.51. Before completing the sale, the seller has to give you a written notice about California’s manufacturing restrictions and collect your signed acknowledgment. You must provide a current ID showing you are at least 18, and the shipping address has to match the address on that ID.2California Legislative Information. Assembly Bill 1263
The package itself must be labeled so the carrier requires a signature and proof of ID from someone 18 or older at delivery. One practical snag: UPS, FedEx, and USPS all define “adult signature” as 21 or older for their signature services. California sets the purchase age at 18, but if you are between 18 and 20, the carrier’s own policy can block delivery even though the sale is legal.
Retailer Refusals
Some online retailers simply will not ship any firearm-related part to California. That is almost always a business decision about compliance cost and liability, not a legal prohibition. Other vendors ship to California routinely. If one refuses, keep looking.
Features That Turn Your Build Into an Assault Weapon
Buying the upper is the easy part. The hard part is making sure the finished rifle does not meet California’s definition of an assault weapon, because the determination looks at the combination of features across the upper and lower.
A semiautomatic, centerfire rifle with a detachable magazine becomes an assault weapon if it has any one of these features: a pistol grip that protrudes conspicuously below the action, a thumbhole stock, a folding or telescoping stock, a grenade or flare launcher, a flash suppressor, or a forward pistol grip.3California Legislative Information. California Penal Code 30515 – Assault Weapon Definition The trigger for all of that is a detachable magazine. California gun owners build around the rule in one of two ways.
Featureless Builds
A featureless rifle avoids every prohibited feature on that list, which lets you keep a standard detachable magazine. In practice, this usually means swapping the pistol grip for a fin grip or similar compliant grip, running a fixed-length stock, and keeping the upper free of a flash suppressor. A muzzle brake or compensator that is not designed to reduce flash is fine. For most people, this is the simplest compliant AR-style configuration.
Fixed-Magazine Builds
The other route is to lock the magazine in place. Under Penal Code 30515, a “fixed magazine” is one that cannot be removed without disassembling the firearm’s action.3California Legislative Information. California Penal Code 30515 – Assault Weapon Definition For an AR-15, that means the magazine cannot come out while the upper and lower are joined.4Legal Information Institute. California Code of Regulations Title 11 Section 5471 Aftermarket devices accomplish this by forcing you to pop the receivers open before the magazine releases. With a fixed magazine, you can keep the pistol grip, adjustable stock, and flash suppressor. Reloading is much slower.
Barrel Length and the Short-Barreled Rifle Trap
The barrel on your upper matters. California defines a short-barreled rifle as any rifle with a barrel under 16 inches, an overall length under 26 inches, or any weapon made from a rifle that falls below those measurements.5California Legislative Information. California Penal Code 17170 – Short-Barreled Rifle Manufacturing, selling, transporting, or possessing one without a dangerous weapons permit is a crime.
You do not have to assemble the rifle to be charged. California treats possession of parts that could be readily assembled into a short-barreled rifle as a violation when those parts are under the same person’s control.5California Legislative Information. California Penal Code 17170 – Short-Barreled Rifle Own an AR-15 lower, buy a 10.5-inch upper, and you now have the components for an illegal SBR. A prosecutor does not need to prove you put them together. Think hard before buying a short-barreled upper for any reason, including use on a registered pistol lower.
Serial Number From the DOJ Before You Assemble
If the upper is going on a lower that does not already carry a manufacturer’s serial number, California requires you to get a serial number from the Department of Justice before assembly. Under Penal Code 29180, anyone manufacturing or assembling a firearm without a valid serial number must first apply to the DOJ for a unique serial number or mark of identification.6California Legislative Information. California Penal Code 29180 – Manufacturing or Assembling a Firearm
The application asks for a description of the firearm you plan to build along with your name, address, and date of birth. Once you receive the number and finish assembly, you have 10 days to engrave or permanently affix it to the firearm, then notify the DOJ that the number has been applied.6California Legislative Information. California Penal Code 29180 – Manufacturing or Assembling a Firearm The number goes on the frame or receiver, which for an AR-15 is the lower. If your lower is a commercially made, serialized unit, you are already covered. If it is an 80-percent lower or any other unserialized frame, the DOJ serial number is not optional.
Magazine Capacity
Possessing a magazine that holds more than 10 rounds is illegal in California, regardless of when you got it.7California Legislative Information. California Penal Code 32310 The limit applies to detachable and fixed magazines alike. If your upper arrives as part of a kit or bundle with a standard 30-round magazine, you cannot legally bring that magazine into the state.
Penalties for a Non-Compliant Build
- Manufacturing or selling an assault weapon is a straight felony carrying four, six, or eight years in state prison.8California Legislative Information. California Penal Code 30600
- Possession of an assault weapon is punishable by up to one year in county jail or a state prison sentence.9California Legislative Information. California Penal Code 30605
- A short-barreled rifle offense is a wobbler, charged as a misdemeanor or felony. The felony version carries up to three years in prison.
- Large-capacity magazine possession is an infraction with a $100 fine per magazine, or a misdemeanor carrying up to one year in county jail and a $100 fine per magazine.7California Legislative Information. California Penal Code 32310
The assault weapon charges are the ones to focus on. A build you thought was compliant can become a felony because of a single wrong muzzle device or an adjustable stock paired with a standard magazine release. If more than one assault weapon is involved, each counts as a separate offense.8California Legislative Information. California Penal Code 30600 Check every feature against Penal Code 30515 before the upper and lower ever come together.