On Friday, California Governor Jerry Brown signed into law legislation requiring anyone who makes or assembles a homemade firearm to apply for a serial number or "other mark of identification" from the state Department of Justice. This means passing a background check. Said measure, one of several new gun restrictions signed by Brown, mandates that said identification be permanently affixed to the weapon, and, moreover, forbids the sale or transfer of self-assembled firearms.
The new law is an overt response to the 3D printing boom as well as the increasing sale of "unfinished" lower receivers. These are somewhat modular gun components encompassing trigger, firing pin, and ammunition feeding mechanisms. While the finished versions of lower receivers have historically been subject to the same laws as regular old long guns, unfinished versions requiring only a few small tweaks have offered a gun buyers a fudge. The new law aims to close this loophole.
"Homemade guns may be of very poor quality and extremely unsafe"
A bit more subtly, the bill goes after would-be undetectable plastic guns, mandating that in order to pass California state muster, the are required to have a piece of stainless steel embedded somewhere such that they'll register in a metal detector.
As for the no sale/transfer clause, an argument put forth on the state Senate floor back in may by the California Chapters for the Brady Campaign to Prevent Gun Violence states: "Homemade guns may be of very poor quality and extremely unsafe and should therefore only be for personal use. At this point, many 3-D printed guns explode when they are fired. The technology will, no doubt, improve but it is unlikely that these guns would ever meet basic firing or drop tests and such unsafe guns should not be transferable." (ATF testing in 2013 found that some popular models do indeed have a habit of exploding.)
Naturally, the NRA and gun rights groups are furious. The president of the Firearms Policy Coalition offered the totally hyperbole-free declaration that "Today's action by Governor Brown shows how craven California's despotic ruling class has become. The Legislature has abandoned the Constitution, representative government, and the People of California. I fully expect the People to respond in kind."
A Field Poll conducted in January found that most California voters, including Republicans, support increased gun control measures, including requiring background checks for purchasing ammunition and outlawing possession of large-capacity magazines, among others.