Threaded barrels are not illegal in Washington State as standalone parts — you can buy, sell, and possess one. The legal problem starts when you attach one to certain semi-automatic firearms, because that combination can classify the gun as an “assault weapon” under the state ban that took effect April 25, 2023. Whether a threaded barrel puts you on the wrong side of the law depends on what it’s mounted to and when you acquired that firearm.
When a Threaded Barrel Turns a Gun Into an Assault Weapon
RCW 9.41.010 defines an assault weapon by feature combinations. A threaded barrel is one listed feature, but it only matters on specific platforms, and only when paired with other characteristics. The barrel by itself doesn’t trigger anything.
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 threading to matter. A rimfire semi-auto with a threaded barrel, or a bolt-action centerfire rifle with one, sits outside this definition.
Semi-Automatic Pistols
A semi-automatic pistol with a detachable magazine qualifies as an assault weapon if its barrel is threaded to accept a flash suppressor, forward handgrip, or silencer. There’s no centerfire requirement for pistols — any semi-auto pistol with a detachable magazine and threaded barrel meets the threshold.1Washington State Legislature. Washington Code 9.41.010 – Terms Defined
Shotguns
Semi-automatic shotguns have their own list of features, and a threaded barrel isn’t 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.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 can’t stock these guns, and you can’t 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 bigger trap for existing owners: threading an existing barrel, or swapping in a threaded one, is treated as manufacturing a new assault weapon. If you own a semi-automatic pistol with a detachable magazine and no threaded barrel, adding threads creates a newly manufactured assault weapon and violates the ban.1Washington State Legislature. Washington Code 9.41.010 – Terms Defined
What You Can Keep From Before the Ban
The ban doesn’t require you to give up guns you already owned. If you legally possessed an assault weapon before April 25, 2023, you can keep it. The law targets new sales and manufacturing, 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 permitted. That’s maintenance on a grandfathered gun, not new manufacturing. The line matters: like-for-like part swaps on a pre-ban firearm are fine, but adding features that push a currently legal gun into assault-weapon territory is not.
Inheritance is possible but restrictive. You can inherit an assault weapon if you can establish it was lawfully possessed before the ban. Once you have it, you can’t sell or transfer it to another private individual — your only transfer options are through a licensed firearms dealer or to law enforcement.1Washington State Legislature. Washington Code 9.41.010 – Terms Defined
Penalties
Manufacturing, importing, distributing, selling, or offering to sell 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 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 rights.
Federal exposure is separate and much heavier. If a threaded barrel is used with an unregistered suppressor or another federally regulated item, penalties run up to 10 years in prison and $250,000 in fines for an individual.4ATF. NFA Handbook – Chapter 15 – Penalties and Sanctions
Suppressors: The Reason Most People Want Threads
Suppressors are legal to own and use in Washington, but only if they’re properly registered under the federal National Firearms Act. Using an unregistered suppressor is its own gross misdemeanor under RCW 9.41.250, stacked on top of any assault weapon violation.5Washington State Legislature. Washington Code 9.41.250 – Dangerous Weapons, Penalty
Federal registration means submitting ATF Form 4, undergoing a background check, and providing fingerprints and a photograph. Each responsible person on a trust or legal entity submits a separate questionnaire, and you notify your local chief law enforcement officer of the pending transfer.6Bureau of Alcohol, Tobacco, Firearms, and Explosives. Application to Transfer and Register NFA Firearm (Tax-Paid)
One recent change matters for cost. As of January 1, 2026, the federal transfer tax for suppressors dropped from $200 to $0. The $200 tax now applies only to machineguns and destructive devices. The paperwork and wait haven’t changed, but there’s no tax payment required for a suppressor transfer.7Office of the Law Revision Counsel. 26 USC 5811 – Transfer Tax
Keep the state rule in mind: even a fully registered federal suppressor doesn’t fix the assault weapon problem. If you thread the barrel on your semi-auto pistol or centerfire rifle with a detachable magazine to attach that suppressor, you’ve manufactured an assault weapon under Washington law.
How to Stay Compliant
The statute defines these feature combinations clearly, and the presence of a threaded barrel plus a detachable magazine on a semi-auto rifle or pistol platform is enough. It doesn’t matter what you actually attach to the threads — a thread protector, nothing at all, or a registered suppressor. The threading is the feature the law counts.
Some owners look at permanently attaching a muzzle device by pinning and welding it — drilling through the device into the barrel threads, inserting a pin, and welding over it so it can’t be removed without destroying the barrel. That method is widely used for federal barrel-length compliance, but whether it removes the “threaded barrel” classification under Washington’s statute is not something the law addresses directly. Talk to a firearms attorney familiar with Washington law before making that kind of modification.
The straightforward path for new purchases is picking a firearm that either lacks a threaded barrel or uses a fixed magazine, since the definition requires both features together. A semi-automatic pistol with a fixed magazine and a threaded barrel, for instance, doesn’t meet the statutory definition.