Earlier this year, Cody Wilson, the 25-year-old founder Defense Distributed, a Texas-based group that promotes the use of 3-D printed guns, fired a .380 caliber bullet from a plastic gun called the "Liberator." The shot landed at a dusty firing range in central Texas, but was apparently heard in the halls of Congress.
The provocative demonstration prompted fears from politicians that criminals would be able to arm themselves in the future by simply printing guns in their basements.
On Monday, the U.S. Senate addressed those concerns by voting to extend the Undetectable Firearms Act for another 10 years, mirroring similar action last week by the House. The legislation, expected to be signed by President Obama, continues the ban on the sale or possession of firearms that aren't detectable by X-ray machines or metal detectors, a category that could include 3-D printed guns.
“In 1988, when we passed the Undetectable Firearms Act, the notion of a 3-D printed plastic firearm slipped through metal detectors, onto our planes in secure environments was a matter of science fiction,” Rep. Steve Israel, D-N.Y., said on the House floor. “The problem is that today it is a reality.”
The law, as extended, requires 3-D guns to have a metal strip that would make them visible to metal detectors. Some Democratic senators wanted stricter controls, including a requirement that 3-D printed guns have permanent metal components. They argued that a non-permanent metal strip could be taken off the gun, allowing it to pass through metal detectors and scanners before being reinserted. But in the end the Senate simply approve the House-passed bill, meaning that regulations for 3-D printed firearms will look pretty much the same tomorrow as they did yesterday.
For 3-D gun proponents like Wilson, the vote was a mixed bag. On one hand, he told NBC News, it means that completely banning the technology was likely "off the table."
"As the technology is adopted and gets more popular, it looks like 3-D printed guns have a future now," he said. But, he also said, he was "not optimistic" about what it meant for 3-D printed guns overall, claiming that it could lay the groundwork for further regulations. "I'm still expecting more restrictions as a result of this law being passed."
MORE: Senate extends ban on undetectable guns but nixes tighter restrictions
It's not clear just how advanced 3-D printed guns can get. In November, the Bureau of Alcohol, Firearms and Explosives (ATF) attempted to build and fire one of Defense Distributed's Liberators, only to watch it explode.
"Will fully plastic handguns will ever be as reliable as regular handguns?" said Wilson, a self-described "crytpo-anarchist" who opposes gun control. "The answer is no, not directly. Down the road, there will be hybrid materials that people might use, but right now, it's more impractical, more experimental."
Local governments have also made moves to regulate 3-D printed guns. In Philadelphia, the city council passed an ordinance that would hit anyone caught with a 3-D printed firearm with a fine of up to $2,000.
While the Philadelphia Police Department has never caught anyone using or making weapons with a 3-D printer, it supports the new law, Lt. Francis Healy, special adviser to the police commissioner, told NBC News.
"It’s a good idea," he said. "It's not like we saw a rash of 3-D printed guns in Philadelphia, but the city council is just trying to be proactive."
State and local legislators in California, New York and Washington, D.C., have proposed similar measures.
This isn't new technology. Large manufacturers have been using 3-D printers to build plastic parts for decades. Over the last few years, however, their cost has come down, making them relatively affordable for people who want to print anything from plastic hangers to 3-D portraits of themselves.
In May, however, Wilson raised the profile of 3-D printing by posting a video of himself firing the Liberator on YouTube, an event that created intense media hype. He didn't break the law, because he put a 6-ounce metal strip in the gun.
Since then, Wilson estimated that CAD blueprints for the gun have been downloaded at least 1 million times, spreading to peer-to-peer file-sharing sites like the Pirate Bay after the U.S. government banned Defense Distributed from letting people download the file from its site.
Most of the legislation in Washington and local governments has centered around punishing people found with 3-D printed guns, not preventing people from printing the guns in the first place.
"There is no way of purging the Internet of these files. It's just like the file-sharing conversation that played out over the last decade," Wilson told NBC News."That's why no legislation is being written to stop it — everyone already understands how difficult it is."
Printing a gun isn't prohibitively expensive. The printer and the plastic combined can be bought for as little as $1,400, Cody said, while an ideal set-up would involve a higher-end 3-D printer that normally sells for around $6,000.
Of course, much more reliable, traditionally manufactured handguns sell for under $400 — cheaper on the black market — and don't require the technical know-how to read CAD files and hook up a 3-D printer.
"The gun laws are so weak in this country, it's hard to imagine that there is any reason for someone to go out and buy a 3-D printer, download these blueprints, and test-print a gun, when even people who are clearly dangerous can get guns, if not through licensed sales, then through private ones," Ladd Everitt, director of communications for The Coalition to Stop Gun Violence, told NBC News.
Everitt warned that despite their relative scarcity, guns that can evade metal detectors are a legitimate concern. As for Wilson and Defense Distributed, Everitt said that he was more concerned about the ideology the organization was spreading as a result of widespread media attention than the guns themselves.
"He has been very honest in saying that the point of promoting 3-D printed guns is to essentially foment insurrectionism," he said, "to send a message to our government and other governments around the world that you cannot regulate firearms because we can print our own if necessary, and if you go too far, we can use them."
Keith Wagstaff writes about technology for NBC News. He previously covered technology for TIME's Techland and wrote about politics as a staff writer at TheWeek.com. You can follow him on Twitter at @kwagstaff and reach him by email at: Keith.Wagstaff@nbcuni.com