An excellent question and definitely something that you want to be armed with knowledge about. And there are several different answers to this I wish it was a lot more straight forward however we do live in the state of California where this is OK if you do this, but this is not OK if you do it like this. Not to mention the rules change every other day sometimes. So there’s a straightforward answer and there’s a long answer and there’s also the pairing answer with muzzle devices. So let’s start with the short answer. Yes threaded barrels are legal in California on rifles.
Now let’s go into the long answer. Why is there rifles that do not have threaded barrels in California when their counterparts in other states do? Well, it starts out with the manufactures decision. It’s well known that California has some of the strictest firearms laws in the country. And let’s go a little further down the rabbit hole with a pairing answer of certain muzzle devices are not allowed in California. We can get a little deeper into that later. So the firearms manufacturer makes a decision of whether or not to thread that barrel and put a muzzle device on it, and if it seems or looks like California is going to push back on them threading the barrel and putting a muzzle device on there to where that rifle is unable to be sold in California; the manufacture simply does not thread the barrel.
Take for example Ruger with a mini 14. There’s several variance of the mini 14, however in California you get the basic ranch rifle with a non-threaded barrel. However in other states they have the exact same ranch rifle with a muzzle device on it. Why won't Ruger just put that same muzzle device with Threaded barrel on The California legal mini 14? The Reason is California has deemed the muzzle device that Ruger was using, to be an illegal muzzle device that they didn’t want in California. Now, I don’t work at Ruger, However if I had to guess, Ruger was frustrated didn’t feel like dealing with California and their ever changing laws; so they just took the Threaded barrel feature and muzzle device feature off of the mini 14 and made that version for Sale in California.
That leads us to the next question, can you have the barrel threaded and put a California legal muzzle device while remaining California compliant? You can, however you were going to have to find a good gunsmith to do that project, prices may vary and not every shop offers gunsmithing services. I’m sure you have questions about the A.R. Platforms, We will post a thread about California compliant A.R. platforms. Please continue to read the next part of this thread as you will need the knowledge on model devices as well. Continuing on with some of the rifles that do not have threads on their barrel it also can be because that is the way the rifle was designed. Some of your bolt rifles (Bolt Action Rifles) and some of your other semi automatic rifles have no threads at the end of the barrel. Not necessarily because of California laws but more so because the manufacture of that firearm did not want to mess with the barrel harmonics. Again these are more seen on your bolt rifles which are normally used for a slower rate of fire and made more for accuracy rather than sustained fire with follow up shots.
There are other manufacturers that put out rifles with a threaded barrel and instead of putting a muzzle device on the firearm, they put a thread protector to protect the threads of the end of the barrel, leaving the option to put a muzzle device on the user/operator. So again back to the short answer yes you can legally have threaded barrels on your rifles in California. Now let’s go to the pairing answer on this of muzzle devices.