3D firearms: What does it mean for gun control and other questions
The online release of software which would allow anyone to 3D-print their own gun is now on hold, after a US federal court ruling on Tuesday.
Gun access advocacy group Defense Distributed wants to make their plans available online so that anyone can make their own weapon - without any experience or background checks.
What is a 3D-printed gun?
Advances in 3D-printing technology have made it feasible to create gun parts using a simple set-up: a mill or plastic printer hooked up to a computer.
Defense Distributed founder Cody Wilson invented the first 3D-printed gun five years ago.
The Liberator, a single-shot gun, was mostly plastic and less accurate than a conventional pistol. But it was printable - and untraceable.
The plastic gun raised concern over how easy it was to sneak past metal detectors.
In 2013, Israeli journalists were able to smuggle a 3D-printed gun based on Defense Distributed blueprints into the country's parliament.
Mr Wilson's company now has a new printer model which allows gunsmiths to create aluminium objects from digital files, with no experience needed.
Aluminium, like plastic, is not a material normally used in guns due to its malleability, which could cause 3D-printed parts to warp when fired.
Gun control advocates were the first to nickname these untraceable firearms "ghost guns", but the term has since been adopted into the pro-gun vocabulary, too.
These guns have no serial number and are illegal to buy or sell, but making one is still legal.
Defense Distributed does not sell any fully assembled weapons - just the data files and printers needed to make them.
The Ghost Gunner 2, Defense Distributed's updated mill, reportedly costs under $1,500, and is already sold out for 2018. A $250 deposit reserves a place on the list for the next batch of machines.
The machine is also "fully programmable", according to Defense Distributed's website, meaning users can code and complete their own projects even if the company does not sell a design file for it.
Is this legal?
In the US, most gun parts have little regulation and are not individually considered as "firearms". These parts can be shipped or sold without a federal firearms licence (FFL).
The lower receiver, the part that essentially holds the gun together, is considered a firearm under US law, but only if it is complete.
An 80% complete lower requires minimal effort to mill into a finished lower, but it is not considered a firearm. This finished lower can be tailored to build different guns, including AR-15s, full-sized rifles or pistols.
It is these lower receivers that Defense Distributed's Ghost Gunner is primarily designed to create.
When buying a gun from a dealer with an FFL, buyers receive an instant background check using a national database.
But with this new technology, gun owners can print and assemble their own firearms, without a serial number or background check.
Defense Distributed notes on their website that some states may have manufacturing restrictions, but as long as users do not sell their homemade firearms, it is legal to print them.
Are there any concerns?
Retired Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) agent David Chipman likens the 3D-printing gun debate to the development of bump stocks, a device that enables a rifle to shoot hundreds of rounds a minute.
"I was at ATF when a company asked us to render an opinion about a bump stock that I can tell you seemed to be a flimsy, cheap device that seemed gimmicky," he told the BBC.
"I can tell you my shock to see that piece of technology transformed into this device that was used to kill dozens and wound hundreds in Las Vegas."
Mr Chipman, a gun owner himself, says he believes Americans who can pass background checks have every right to a gun, whether they purchase it or build it. It's the lack of any regulation that worries him.
"We understand that you probably can make your own car, but if you take it on the street, it's got to be licenced," Mr Chipman says.
"We have to make sure the person making [the gun] isn't a criminal and we have to make sure if that gun is stolen or otherwise used in crime, that it has markings allowing police to trace it."
And that's the biggest question around these printable guns: they are potentially accessible to felons or those who otherwise would not pass the basic background checks at gun dealerships.
Mr Chipman said traceable weapons are key to solving cases, and the prospect of ghost guns is a "real threat".
"Being able to trace a gun back to who purchased it is a core piece of crime solving that we rely on," he said.
"I don't know if it's a threat today or tomorrow, but five years from now - I can guarantee it."
Avery Gardiner, the co-president of the Brady Campaign gun control advocacy group, said the organisation will not back down in the fight against 3D-printed guns.
"Ever since Cody Wilson unveiled his 3D-printed gun years ago, it's been clear to us what a threat this poses not just to American citizens, but to people around the world," Ms Gardiner said in an emailed statement to the BBC.
"These blueprints will end up in the hands of the most dangerous people among us, from domestic abusers and convicted felons to even drug cartels and terrorist cells. We are committed to doing everything we can to put an end to this," she said.
"This is as important as it gets."
What do gun advocates say?
Gunsmithing in America is not a new hobby. A quick Google search pulls up YouTube videos of making gunsmithing drill presses, and with a little mechanical know-how, it's entirely possible to build AR weapons.
And, as many DIY enthusiasts are saying online, it's fairly time-consuming to mill and assemble a weapon even with the Ghost Gunner.
Paul Sheley, a gun enthusiast in Portland, Oregon, says he downloaded Defense Distributed's files in 2013 "within the hour" they were released.
"I intend to use these files, but what I further intend to do is disseminate them," he told the BBC. "I want everyone to have it therefore nobody can control it."
It's this sense of liberty that has made Defense Distributed's products resonate with much of the pro-gun community.
Mr Sheley, 31, says that while the firm's technology is not new or particularly groundbreaking, it's making a statement.
"I think what they're actually trying to do is make the point that gun control is a futile effort and it's never going to stop murder," he says. "I feel much safer having my own [gun] than being disarmed totally - and criminals will never disarm.
"There's always the human element. The whole taking a file off the internet and punching in some digits on a computer and then having it pop out of a printer - it's never that simple."
The lobby group Gun Owners of America (GOA) also described 3D-printing guns as "freedom" in a statement.
"Our rights do not depend on what criminals do," Erich Pratt, executive director, said.
"Already, almost 80% of firearms used in crime are weapons that were not legally owned."
A 2016 study by the University of Pittsburgh found that 79% of criminals were "not the legal owner" of the firearms they used, pointing to theft or trafficking as the main source of "bad guy" guns.
What does this mean for US gun control?
Defense Distributed's enterprise had been contested by the Obama administration, but President Donald Trump's justice department ruled this summer that Americans may "access, discuss, use and reproduce" the technical data.
On Tuesday, President Donald Trump tweeted scepticism over plastic guns but it is unclear whether the president will make any changes affecting 3D-printing guns in general.
The NRA later released a statement pointing out that undetectable plastic guns have been illegal for 30 years.
US firearms must contain at least 106g of metal, but Ghost Gunner's aluminium gun parts have no serial numbers and cannot be traced by law enforcement.
Stanford Law Professor John Donohue says these new tools are "a step into the unknown" for the US.
He told the BBC the Trump administration has "created an opportunity for extending this technology to anyone who wants it".
"In the normal context when you're dealing with regulation, you're dealing with physical entities and maybe gun merchants and so there is a geographic space that states can more easily command," Prof Donohue says.
"When you're talking about the internet, there are no geographic parameters to govern, so it really is a difficult thing for the individual states to police."