In the past three years, this model of homemade semiautomatic firearm, known as an FGC-9, has appeared in the hands of paramilitaries in Northern Ireland, rebels in Myanmar and neo-Nazis in Spain. In October, a British teenager will be sentenced for building an FGC-9 in one of the latest terrorism cases to involve the weapon.

An online group known as Deterrence Dispensed publishes free instructions on how to build the weapon, a manual that says people everywhere should stand armed and ready.

“We together can defeat for good the infringement that is taking place on our natural-born right to bear arms, defend ourselves and rise up against tyranny,” the document says.

This American brand of libertarianism has historically been a tough sell in many other parts of the world. Even if some people believed it in theory, strict laws made buying a gun so difficult that the ideology was almost beside the point.

The FGC-9 is changing that.

“It’s not just a gun. It is also an ideology,” said Kristian Abrahamsson, an intelligence officer with the Swedish customs police. Dozens of FGC-9s have turned up in his country in recent years, he said.

The New York Times has charted the FGC-9’s growth from a hobbyist’s garage project to a lethal pistol wielded by insurgents, terrorists, drug dealers and militia members in at least 15 countries.

While countless 3D-printed guns have been designed and circulated on the internet, international law enforcement officials say that the FGC-9 is by far the most common. The gun is so desirable among far-right extremists in Britain that the possession and sharing of its instruction manual is being charged as a terrorist offense.

Nobody does more to promote the gun and the ideology than its co-designer, who goes by the online name Ivan the Troll. The figurehead of Deterrence Dispensed, he has appeared in numerous YouTube videos and podcasts, but always under his aliases.

Court documents, corporate records and information posted on his social media accounts link the Ivan the Troll persona to a 26-year-old Illinois gunmaker named John Elik. The nephew of a state representative, Mr. Elik has emerged as one of the most important figures in the nascent international industry of 3D-printed guns.

Police forces around the world see that industry as a threat to gun restrictions that have limited who can have access to firearms. On that, the authorities and gun rights supporters agree.

The Times sent an interview request and an article summary to Mr. Elik’s email address. A reply from an Ivan the Troll account declined to answer questions and said that he did not believe he would be treated fairly.

In the United States, 3D-printed guns are regulated by a hodgepodge of state laws. Illinois has restricted the sale and possession of homemade gun components, except by firearm dealers and manufacturers. Because he is a licensed manufacturer, there is no indication that Mr. Elik is violating that law. Illinois law requires manufacturers to add serial numbers to homemade gun components.

The videos posted online encourage viewers to know their local laws.

Most of the mass-produced weapons of the 20th century, even those now marketed for personal defense, were originally designed for militaries and hunters. The FGC-9, by contrast, was created with the explicit goal of arming as many everyday people as possible.

FGC is an abbreviation that represents what its creators think of gun control. Nine is for the 9-millimeter bullet it fires.

The use of the FGC-9 by insurgents opposed to the military junta in Myanmar is part of its creators’ stated plan, a realization of the hope that guns could be used to stand up to the state.

Mr. Elik, in his email to The Times, said it was wrong to focus on “European cops complaining about a small number of guns being recovered,” and shootings in which nobody was injured, “rather than the gun’s use as a tool of liberation.”

The Prototype

The gun’s chief designer was Jacob Duygu, a German national of Kurdish heritage.

Germany requires gun owners to be licensed, but Mr. Duygu wanted to own a firearm on his own terms. He made it his mission to give anyone the tools to do the same, especially in countries with strict gun control laws.

Mr. Duygu developed an affinity for American libertarianism and the Second Amendment right to bear arms, according to Rajan Basra, a senior fellow at the International Center for the Study of Radicalization who has studied the proliferation of the FGC-9.

Mr. Duygu was known online as JStark, in homage to Maj. Gen. John Stark, a military leader of the American Revolution.

Social media accounts linked to Mr. Elik have voiced similar views.

“Civilians need assault weapons because having a weapon made for killing people is very important for self defense,” one post read, adding, “If it’s made for killing people quickly and easily, even better.”

Mr. Duygu’s design was published in March 2020 with the stated goal of circumventing gun laws. Homemade firearms have been around for centuries, but Mr. Duygu’s was a breakthrough. The FGC-9 could be built entirely from scratch, without commercial gun parts, which are often regulated and tracked by law enforcement agencies internationally.

Anyone with a commercial 3D printer, hundreds of dollars in materials, some metalworking skills and plenty of patience could become a gun owner.

“It’s a game changer,” Dr. Basra said. “Now you have something that people can make at home with unregulated components. So from a law enforcement perspective, how do you stop that?”

The assembly instructions for the first iteration of the FGC-9 called for each gun to be etched with General Stark’s motto, “Live Free or Die.”

The release of the FGC-9 inspired gun enthusiasts to suggest their own modifications. Among them was Mr. Elik, who separately developed a do-it-yourself process for making the spiral grooves, or rifling, inside a gun barrel.

A year later, a new version of the FGC-9 was released, crediting Mr. Elik’s pseudonym as a co-creator.

Dr. Basra and a security researcher, Nathan Mayer, first linked Mr. Elik to the Ivan the Troll accounts using online clues after he was identified in a lawsuit as an owner of a website promoting 3D-printed guns. The Times replicated and built on that research, using photographs and videos that Mr. Elik posted of his home and shooting ranges on his family’s property, including his aunt’s.

His aunt, Amy Elik, is a Republican state representative and a staunch supporter of gun rights. She voted against the state’s ban on homemade firearms. She did not respond to a message seeking comment.

Mr. Duygu was found dead in 2021 of undetermined causes just days after he was questioned by the German police. Mr. Elik quickly became the highest-profile spokesman for the gun they created and went on to develop his own designs for partly 3D-printed weapons, including versions of a Kalashnikov and of an MP5 submachine gun.

“This pure marketplace of ideas wasn’t created by accident, even if it’s wildly more successful than Stark or I ever imagined,” one of the accounts linked to Mr. Elik wrote online.

Going Viral

The weapon first received high-profile attention in December 2020, when Matthew Cronjager, a British neo-Nazi, was arrested and accused of trying to recruit and arm a militia. The targets included the British government, Jews, gay people, Muslims and members of ethnic minority groups.

Mr. Cronjager, then 17, had downloaded a Deterrence Dispensed manual for making 9-millimeter ammunition and the plans for the FGC-9. He was arrested after trying to pay an undercover officer to manufacture the gun. Mr. Cronjager, who was later convicted and jailed for more than 11 years, said that he wanted to topple the government and start a revolution, court records show.

Since then, several people with white-supremacist and anti-immigrant leanings have been prosecuted for terrorism offenses in Europe after trying to obtain the weapon to commit mass shootings. Drug gangs and prison inmates in Brazil have also been found with the weapon, the authorities there say.

No law enforcement official has yet linked an FGC-9 to a homicide, though they say that may be because traditional forensic techniques are not always reliable on homemade weapons.