One recent grey, rainy morning outside Munich, Ernst Mauch showed a visitor around his office, which sits next to a suburban shopping centre. A conference table was arranged with coffee, plates of cookies and product samples – in this case, pistols.

Mr. Mauch, 58, has spent his entire working life designing deadly weapons. His creations include handguns, machine guns and grenade launchers. The assault rifle he developed is used by elite U.S. special-forces units and is credited with killing Osama bin Laden. To test his firearms, Mr. Mauch would fire thousands of rounds of ammunition in extreme conditions, from deserts in Arizona to Swedish permafrost.

A little over a decade ago, he embarked on a different kind of project. Was it possible, he wondered, to design a safer gun? A gun that could only be used by its owner? In short, a gun that was a little less, well, dumb? "I'm allowed to make that comment," he says with a smile.

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Mr. Mauch succeeded in making a "smart" gun. But the story of why it's not for sale in the U.S. is a long and strange one. It's a tale of a cultural clash between Europe and the U.S. – and of an American gun debate so radioactive that even promising new technologies cannot find a way to market.

The controversy is mystifying to Mr. Mauch, a courtly, barrel-chested man from southern Germany. "I was hoping that people understand the potential," he says. "The only intention [was] to stop and minimize killings of people who do not know how bad these kind of products can be."

'I have one talent'

Mr. Mauch's story begins in a village called Dunningen near Germany's Black Forest, where his father farmed wheat, corn and potatoes. Drawn to intricate machinery, he completed a special course in watchmaking as a teenager then went on to study engineering.

The program required him to get an apprenticeship, but he had troubling finding one. Then his father remembered that a neighbour worked for Heckler & Koch, a German gun manufacturer founded in 1949 and based nearby. Mr. Mauch began working there as a student, then joined as a designer upon his graduation in 1978.

"I have one talent," he says. "I have a feel for small parts, moving parts … things like you see in a watch movement. You can transfer it to a gun, to a pistol."

Mr. Mauch spent 27 years at Heckler & Koch, with stops as head of production design, director of research and development and finally co-chief executive. His customers included the German, British and American militaries. In 2002, he became the first foreigner to win the Chinn Award, an honour presented annually by the National Defense Industrial Association in the U.S. for contributions to small-arms design.

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"He's one of the most remarkable designers of small arms that I've ever known," said David Grange, a retired U.S. Army general who has known Mr. Mauch since 1985. "He's a genius."

In the late 1990s, Mr. Mauch was asked to testify via conference call in a trial under way in Los Angeles. A six-year-old boy had shot and killed his friend, also 6, with a gun belonging to his father and made by Heckler & Koch. For several hours, Mr. Mauch explained how the pistol worked. Newly married, he returned home dejected. He told his wife it had been "a horrible time. To explain that a product you use normally for your own protection killed a young boy."

That experience "was more or less the starting point for me to think" in a different direction, he says. According to the latest statistics from the U.S. Centers for Disease Control, nearly 600 people are killed in accidental shootings in the United States annually, about 100 of them children. An investigation in 2013 by The New York Times found that those statistics may undercount by half the true number of accidental shootings involving children.

For Mr. Mauch, professional pride was also part of his motivation. At annual gun shows, he saw weapons manufacturers from all over the world capable of producing similar products. It was time, he felt, to make a new generation of guns. In an era that has seen the advent of laptops, digital recorders and iPhones, the "killing instruments are still the same," he says.

A corporate battle over the future direction of Heckler & Koch pushed Mr. Mauch toward the exit in late 2005. He says he fielded lucrative job offers from gun manufacturers in six countries, but chose instead to join a small, Munich-based startup called Armatix in 2006. "I did not want to make the stupid guns again," says Mr. Mauch, now the company's managing director.

Introducing a smarter weapon, however, would turn out to be harder than he ever imagined.

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'Just a piece of composite'

There is a black watch strapped to my wrist and a pistol in my hand. Fleeting thoughts of the latest James Bond movie pass through my mind, only I have the ability to fire this weapon. I aim toward the target and squeeze the trigger five times. "Want to fire again?" asks an Armatix employee.

We're in a shipping container that has been converted into a small shooting range. It sits in the back of a parking lot in a Munich suburb, steps from Armatix headquarters. The occasion is a demonstration of the company's iP1 pistol, a .22-calibre handgun and the first "smart gun" for commercial sale.

Mr. Mauch and his team developed a weapon that works using radio-frequency identification – the same technology employed in anti-theft tags on clothes in department stores. To fire its gun, you use an accompanying watch. When that watch is activated with a code and sitting on your wrist – or anywhere less than 25 centimetres away from the gun – the gun will fire. Otherwise, it's a "just a piece of composite," says Mr. Mauch, and useless as a weapon.

The quest to create smart guns, or personalized guns, is not new. A subsidiary of the American gun maker Mossberg Group made a personalized rifle back in 1998 – it works using a ring to activate it – but decided there wasn't a market for it. Other American gun manufacturers such as Smith & Wesson and Colt Manufacturing have also explored developing their own smart-gun technology.

The interest in such safety measures increased after the massacre of 26 people, 20 of them children, at an elementary school in Newtown, Conn., in 2012. The White House ordered the U.S. Department of Justice to conduct a study of the current state of the technology.

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The report, completed in 2013, notes that state authorities in California had subjected the Armatix pistol to mandatory firing and safety tests. It passed them all, including one requiring the pistol to fire 600 rounds with fewer than six malfunctions.

Besieged by critics

By early this year, Armatix was ready to introduce its pistol in the United States. It had marketing materials and promotional swag. Two gun dealers – Oak Tree Gun Club in California and Engage Armament in Maryland – agreed to carry the iP1, which would sell for $1,800 (U.S.).

But it turned out that Armatix had stepped into a minefield. The two gun stores were besieged with criticism from gun-rights enthusiasts, who learned of the product from media coverage and online forums. They flooded the Facebook pages of both stores with angry comments; some even called in death threats, according to a video later posted by Andy Raymond, an owner of Engage Armament.

Part of the reaction stemmed from a little-known New Jersey law passed in 2002 by gun-control advocates. Once a personalized weapon went on sale in the United States, the law said, then within three years, all guns sold in New Jersey would have to include such safety features. Gun-rights activists feared the Armatix pistol would trigger the law's mandate. But they also had a visceral reaction to technology they felt could open the way to government interference with their weapons.

Confronted with an onslaught, both gun stores backpedalled. Mr. Raymond of Engage Armament initially resisted, saying it was a matter of customer choice. He retreated after a night spent sleeping in his store to protect it from perceived threats.

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For Mr. Mauch back in Germany, it was a bewildering disaster. Ordinary Americans, he felt, would understand the value of the product. But "the John Wayne-type guys, they always want to have the gun ready to kill," he says, putting his hand on his hip to simulate drawing a weapon. "They feel – what I don't understand – so threatened."

A former long-time colleague of Mr. Mauch's in the United States, who asked not to be named, called him a "first-rate" weapons designer but said he didn't appreciate the American context. "The thing that worries me and millions like me is that the anti-gunners in our [government] … ONLY want this technology so they can restrict the rights of law-abiding gun owners," the colleague wrote in an e-mail. "Would you want to bet your life on your smart phone or laptop? Me neither."

Those views echo the positions of the National Rifle Association and the National Shooting Sports Foundation, two powerful groups representing gun-rights activists and the gun industry. The NRA is "opposed to government mandates that require the use of expensive, unreliable features," according to a statement posted by its lobbying arm last year. Smart guns open "the door to a ban on all guns that do not possess government-required technology."

Michael Bazinet, a spokesman for the National Shooting Sports Foundation, said the firearms industry does not oppose the development of smart-gun technology, only any law mandating its use. "The marketplace should decide," he said.

Mr. Mauch agrees. He says that Armatix offered to license its technology for free for a limited period of time to U.S. gun manufacturers so they could try incorporating it into their products, but they declined. Four major American gun companies did not respond to requests for comment.

"The gun industry in America is decades behind where it should be in safety technology," said Jonathan Lowy, a long-time lawyer at the Brady Center to Prevent Gun Violence. "The Armatix gun will save lives."

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'This is your mission'

The impasse in the United States was dispiriting for Mr. Mauch and his 30-odd colleagues at Armatix, who are split between the headquarters and a production facility near Jena, in eastern Germany. It's there that the company set up machinery capable of churning out thousands of smart guns. It did an initial run of 5,000 pieces, a "limited edition," says Mr. Mauch, destined for the U.S. market and now sitting idle. The project has already cost the company's financial backer millions of euros.

Sometimes, Mr. Mauch says, he feels like leaving it all behind. His wife and three children push him to continue, telling him, "this is your mission, you have to do it." He is a religious man and his faith animates him to use his talents to help people, he says. "We are not allowed to give up."

Mr. Mauch is developing a larger, 9-mm calibre smart gun more suited to law-enforcement personnel, dubbed the iP9. It will be able to operate in both "smart" and "conventional" modes, he said, and will be able to communicate information on when and where it is fired.

Mr. Mauch is hoping police departments in the United States will give it a try. In early 2015, he'll travel to various cities on a trip organized by a group called the Metro Industrial Areas Foundation, a network of multifaith, community-based organizations. Law-enforcement officials "will understand that he's serious and knows what he's talking about. He respects police greatly," said Rabbi Joel Mosbacher, who is leading Metro IAF's gun-safety initiative.

Meanwhile, in early December, the attorney-general of New Jersey issued a report linked to its smart-gun law, the one which had infuriated gun-rights groups. The state attorney-general determined that the Armatix weapon did not meet the statutory definition to trigger its provisions, because the gun couldn't entirely prevent an unauthorized user from firing it (for instance, if someone kept the gun very close to the authorized user wearing the watch).

For Armatix, the report could represent an opening. Because its smart gun doesn't activate the law's three-year mandate, then opponents in theory have less reason to block the sale of the weapon. Still, Mr. Mauch doesn't believe the landscape has changed significantly: Gun dealers remain wary and hostility remains.

Yet Mr. Mauch, who still lives in the village where he was born, farming a field of wheat, isn't done with his crusade. He wants his weapons to work without fail when they're supposed to, with the deadly force that soldiers and police officers require. But he wants to be equally confident his guns don't work if misused.

He was in his office when he heard about the Newtown massacre two years ago. "Don't misunderstand me, this was a higher number of schoolboys and girls, but only one is too much," he says. "The bad side of the story is that so many killings happen every day and the community does not want to hear it. That hurts me, that hurts my heart."