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Today, California Gov. Jerry Brown signed Assembly Bill 857 into law, requiring Californians who build their own firearms to apply for a state-issued serial number. Previously, guns assembled from parts kits officially flew under the radar. No background checks were required, and no serial number had to be stamped into the finished firearm, making them effectively untraceable.

In 2013, I attended a “gun build party” in southern California, in which I and a dozen others built AK-47s and other Kalashnikov variants from parts kits. My AK, according to the host of the build party, was an Egyptian “Maadi.” Its parts had traveled to the United States by way of Croatia, which most likely received the weapon some time during the Yugoslav wars. He told me that often parts kits come from former conflict zones, and that sometimes the wooden stocks have tally marks notched in them. From my Mother Jones story about the gun build party:

Although US customs laws ban importing the weapons, parts kits—which include most original components of a Kalashnikov variant—are legal. So is reassembling them, as long as no more than 10 foreign-made components are used and they are mounted on a new receiver, the box-shaped central frame that holds the gun’s key mechanics. There are no fussy irritations like, say, passing a background check to buy a kit. And because we’re assembling the guns for our own “personal use,” whatever that may entail, we’re not required to stamp in serial numbers. These rifles are totally untraceable, and even under California’s stringent assault weapons ban, that’s perfectly within the law.

Now that’s no longer the case in California. Homemade weapons have long been a pastime for gun enthusiasts, but some law enforcement agencies have become concerned as they’ve started showing up more frequently at crime scenes.

As the Los Angeles Times reports, today’s bill is part of a more sweeping package of gun safety proposals that California Democrats recently pushed through, including a ban on semiautomatic assault rifles with detachable magazines and requiring background checks for ammunition purchases. Brown signed several of these bills earlier this month, which has been met with an effort to overturn them.