Threaded barrels are not illegal to own in Washington state as standalone parts. No statute prohibits buying, selling, or possessing a threaded barrel by itself. The legal problem starts when you install one on certain semi-automatic firearms, because that combination can classify the gun as an “assault weapon” under the ban Washington enacted on April 25, 2023. Whether a threaded barrel creates a violation depends entirely on what firearm it sits on and when you acquired that firearm.
When a Threaded Barrel Makes a Firearm an Assault Weapon
Washington’s assault weapon law, codified at RCW 9.41.010, lists feature combinations that convert an otherwise legal semi-automatic firearm into a prohibited assault weapon. A threaded barrel is one of those features, but it only matters in combination with other characteristics, and only on certain platforms.
Semi-Automatic Rifles
A semi-automatic centerfire rifle with a detachable magazine becomes an assault weapon if it also has a threaded barrel designed to accept a flash suppressor, sound suppressor, or muzzle brake. The rifle has to be both centerfire and magazine-fed for the threaded barrel to count. A rimfire semi-auto with threads, or a bolt-action centerfire with threads, falls outside the definition.
Semi-Automatic Pistols
A semi-automatic pistol with a detachable magazine qualifies as an assault weapon if it has a threaded barrel capable of accepting a flash suppressor, forward handgrip, or silencer. There is no centerfire requirement on the pistol side. Any semi-auto pistol with a detachable magazine and a threaded barrel meets the threshold.1Washington State Legislature. Washington Code 9.41.010 – Terms Defined
Shotguns
Semi-automatic shotguns are treated under a different list of features, and a threaded barrel is not on it. The shotgun definition focuses on folding or telescoping stocks, pistol grips, thumbhole stocks, forward grips, and fixed magazines holding more than seven rounds. A semi-automatic shotgun with a threaded barrel does not become an assault weapon based on that feature alone.2Washington State Legislature. Washington Code Chapter 9.41 – Firearms and Dangerous Weapons
Adding a Threaded Barrel Counts as Manufacturing
Once a firearm meets the assault weapon definition, Washington law prohibits manufacturing, importing, distributing, selling, or offering it for sale within the state. Dealers cannot stock these guns, and you cannot buy a new semi-automatic rifle or pistol with a threaded barrel and detachable magazine from a Washington retailer.3Washington State Legislature. Washington Code 9.41.390 – Assault Weapons, Manufacturing, Importing, Distributing, Selling Prohibited, Exceptions, Penalty
The piece that catches people off guard: putting a threaded barrel on a semi-auto you already own counts as manufacturing a new assault weapon. If you have a semi-automatic pistol with a detachable magazine and no threaded barrel, threading that barrel or swapping in a threaded replacement creates a newly manufactured assault weapon in violation of the ban.1Washington State Legislature. Washington Code 9.41.010 – Terms Defined
Guns You Already Owned Before the Ban
The law does not force you to surrender firearms you owned before April 25, 2023. If a gun classified as an assault weapon was lawfully in your possession before that date, you can keep it. The ban targets the commercial pipeline, not existing ownership.3Washington State Legislature. Washington Code 9.41.390 – Assault Weapons, Manufacturing, Importing, Distributing, Selling Prohibited, Exceptions, Penalty
Replacing a worn or damaged threaded barrel on a firearm that was already an assault weapon before the ban is maintenance, not manufacturing. The classification doesn’t change. The distinction matters: swapping a like-for-like part on a grandfathered gun is allowed, while adding features that push a non-assault-weapon into that category is not.
Inheritance has its own rules. You can inherit an assault weapon, but you need to be able to show the firearm was lawfully possessed before the ban. After inheriting it, you cannot sell or transfer it to another private individual. The only lawful transfer options are through a licensed firearms dealer or by turning it over to law enforcement.1Washington State Legislature. Washington Code 9.41.010 – Terms Defined
Penalties
Manufacturing, importing, distributing, selling, or offering for sale an assault weapon in Washington is a gross misdemeanor.3Washington State Legislature. Washington Code 9.41.390 – Assault Weapons, Manufacturing, Importing, Distributing, Selling Prohibited, Exceptions, Penalty Under Washington’s sentencing structure, that carries up to 364 days in jail and a fine of up to $5,000. A conviction also creates a record that can affect future firearm possession.
Federal consequences stack separately. If a threaded barrel is used with an unregistered suppressor or other federally regulated item, federal penalties run up to 10 years in prison and fines up to $250,000 for an individual.4ATF. NFA Handbook – Chapter 15 – Penalties and Sanctions
Suppressors in Washington
Most people thread a barrel so they can run a suppressor, so the suppressor rules matter. Suppressors are legal to own and use in Washington, but only if they are properly registered under the federal National Firearms Act. Possession or use of an unregistered suppressor is a separate gross misdemeanor under RCW 9.41.250, on top of any assault weapon charge.5Washington State Legislature. Washington Code 9.41.250 – Dangerous Weapons, Penalty
Staying Compliant
If you own a semi-automatic rifle or pistol with a detachable magazine, your options for adding or keeping a threaded barrel are narrow. The statute defines the triggering combination clearly, and a threaded barrel plus a detachable magazine on a semi-auto platform satisfies it regardless of what muzzle device you actually attach.
Some owners consider permanently attaching a muzzle device over the threads by pinning and welding. The pin passes through the device into the barrel, and weld over the pin makes removal impossible without destroying the barrel. This method is widely used for federal barrel-length compliance, but whether it eliminates the “threaded barrel” classification under Washington’s statute is a separate question the law does not directly answer. Talk to a firearms attorney before cutting or welding anything.
The cleaner path for a new purchase is choosing a firearm that lacks one of the two triggering features. The assault weapon definition requires the threaded barrel and the detachable magazine together, so a semi-automatic pistol with a fixed magazine and a threaded barrel, for example, does not meet the definition.
Status of the Ban Today
Washington’s assault weapon ban has been challenged in court, most recently by the Silent Majority Foundation and several gun sellers in Thurston County Superior Court. As of late 2025, Judge Christine Schaller upheld the law, and the challengers have said they will appeal. The ban is fully in effect while that litigation continues, so every restriction above applies right now.1Washington State Legislature. Washington Code 9.41.010 – Terms Defined