Here at California Tactical Academy, we get a great many questions about what rifles are legal to own, what rifles are assault weapons, and what rifles need to be registered. I will admit that I find the laws convoluted as much as anyone else. Let me try and simplify the laws and definitions as much as I can.
Currently Assault Weapons are defined two ways. Initially, California developed a list of firearms, by name, which were deemed too dangerous for citizen ownership. As such certain rifles and pistols were banned, such as the Uzi, Colt AR-15, AK-47 and a variety of other guns. Firearms manufactures responded by creating new rifles which looked and functioned exactly like the banned rifles but were called a different name. And consumers purchased them, thus bringing them into the market stream. Next California legislators banned certain features. The following features are banned on semi-automatic firearms, which fire a center fire cartridge, and are equipped with a detachable magazine;

Flash Hider
Bayonet Lug (by way of Federal law, not State law)
Forward Pistol Grip
Grenade Launcher
Pistol Grip
Thumbhole Stock
Folding Stock
Collapsible Stock

Predictably, the market place responded to the continued demand for semi-automatic, centerfire rifles, with removable magazines and the so called evil features. The Bullet Button device and similar magazine catches were created. These devices replaced the magazine release on previously banned rifles, thus creating a “fixed” magazine which required a tool to remove and exchange the magazine. Following a shooting where the suspects used illegally modified rifles, which had prior to modification used bullet button style devices, the California Legislator expanded the definition of detachable magazines, to include Bullet Button type devices, thus making previous legal rifles, now Assault Weapons. People who own rifles equipped as such have until the end of June 2018 to register their firearms as Assault Weapons with California DOJ. The alternative to registration is to make the rifle a “featureless rifle” by removing the features listed above. You may posses and unregistered rifle in semi-automatic configuration, with detachable magazine if you do not have the features listed above. The simplest way of doing this is to replace the pistol grip with one that has a “fin” like configuration that does not allow a full, opposed thumb grip, ensuring the stock will not collapse or fold, and using a muzzle brake in lieu of a flash hider. If you have a grenade launcher or forward pistol grip, you should also remove these items for compliance.
Keep in mind, that these laws apply only to semi-automatic, magazine fed, center fire rifles. If you own a magazine fed, bolt action rifle, you can have all the other features. If you own a rimfire (think .22 caliber) rifle, you can have the outlawed features, and if you have a rifle with a fixed magazine (of no greater than 10 rounds) you can have all the outlawed features.
I know these laws can be confusing and daunting. I am a Law Enforcement veteran with both municipal and federal Law Enforcement experience, in addition, I have a Law Degree. Even still, I sometimes find the laws difficult to keep up with. Please do not hesitate to ask one of our staff if you have any questions about how to comply the law, while still enjoying your firearms. We are happy to help. And we hope to see you at the range.