A man in Wisconsin viewed it as a technical challenge. Another, in New Hampshire, was looking to save some money. And in Texas, a third wanted to make a political point.

The three may have had different motivations but their results were the same: Each built a working gun that included a part made in plastic with a 3-D printer.

What they did was legal and, except for the technology and material used, not much different from what do-it-yourself gunsmiths have been doing for decades. But in the wake of the shootings in Newtown, Conn., and the intensified debate over gun control, their efforts, which began last summer, have stoked concerns that the inexpensive and increasingly popular printers and other digital fabrication tools might make access to weapons even easier.

"We now have 3-D printers that can manufacture firearms components in the basement," said Rep. Steve Israel, D-N.Y. "It’s just a matter of time before a 3-D printer will produce a weapon capable of firing bullets."

A 3-D printer builds an object layer by layer in three dimensions, usually in plastic. To effectively outlaw weapons made with them, Israel wants to extend an existing law, set to expire this year, that makes weapons that are undetectable by security scanners — like a printed all-plastic gun — illegal.

But there are also major technical obstacles to creating an entire gun on a 3-D printer, not the least of which is that a plastic gun would probably melt or explode upon firing a single bullet, making it about as likely to kill the gunman as the target.

In the meantime, Michael Guslick in Milwaukee, Chapman Baetzel in Dover, N.H., and Cody Wilson in Austin, Texas, did something much simpler and, for now, more effective. They printed the part of an AR-15 assault rifle called the lower receiver, the central piece that other parts are attached to. Then, using standard metal components, including the chamber and barrel — the parts that must be strong enough to withstand the intense pressure of a bullet firing — they assembled working guns.

In all, the three men, who have written about their efforts on the Web, have fired hundreds of rounds, although the plastic receivers eventually deform, crack or otherwise fail from heat and shock. But Wilson, for one, is working on a fourth-generation design that he says should be more durable.

A lower receiver is the only part of an AR-15 that, when bought, requires the filing of federal paperwork. But it is legal to make an AR-15 — and many other guns — for personal use as long as there is no intent to sell it. And if the lower receiver is homemade, no paperwork is required.

Amateur gunsmiths have made lower receivers for years, in metal, although the process requires a certain level of machining expertise. Inexpensive 3-D printers have grown in popularity — their rise has been compared with that of personal computers in the 1980s — in part because they are easy to use. It is not even necessary to know how to create the design files that instruct the device to print bit after bit of plastic to build the object, as there are files for tens of thousands of objects available on the Internet, created by other users and freely shared.

Still, some tinkering is usually required. Guslick, who works in information technology and describes himself as a hobbyist gunsmith, printed his receiver on a machine he bought online through Craigslist. He used a file and abrasive paper to make the piece fit properly, but overall the project was not much of a technical challenge.

"Anybody could do this," he said.

Baetzel, who made his receiver on a 3-D printer he built from a kit, said the part worked fine until he cracked it when bumping the gun while putting it in his car. He has since printed a replacement along with a modified grip and stock which, he said, has made the gun sturdier.

For Baetzel, who works as a software tester, the motivation for printing gun parts was economic.

"Shooting is an expensive sport," he said.

He figured he could save perhaps $40 by making the receiver rather than buying one.

Only Wilson, a law student who prints his receivers on friends’ machines, had overtly political motives, wanting to demonstrate what he called the absurdity of gun-control laws. He took his efforts even further, printing high-capacity magazines like those that would be banned under recommendations proposed by President Barack Obama and successfully testing them earlier this month on a firing range south of Austin. He has posted the drawing files at his website, defcad.org, so that others can print the magazine.

"It’s unbannable," he said. "The Internet has it now."

Wilson also has a project to develop a fully printable one-shot weapon, although he has not made much progress. He is seeking a firearms manufacturer’s license, which he would need to even make prototypes of a complete weapon.

He gets advice and technical help from a loose network of about 15 collaborators around the world and has posted other printer files at his site, including Guslick’s file for a lower receiver.

Baetzel posted his files on his own blog, Ambulatory Armament Depot, after a printer file-sharing site, Thingiverse, forced him to remove them in December. A spokeswoman for MakerBot, a 3-D printer manufacturer that sponsors Thingiverse, pointed out that the site’s terms of service prohibit content that "contributes to the creation of weapons."

Guslick, who is machining a couple of metal lower receivers, said 3-D printers were far from the best tool for gun-making, an opinion shared by Neil Gershenfeld, a professor at MIT and director of the school’s Center for Bits and Atoms.

"A well-equipped machine shop for a long time has been able to make gun parts," Gershenfeld said. "Three-D printers make not very good ones."

The types of computer-controlled tools found in a machine shop — primarily laser cutters and milling machines — are expensive. But smaller and cheaper versions are now available to dedicated hobbyists, although they do not yet have quite the mass appeal of 3-D printers.

Yet the printers have other drawbacks besides the use of plastic. They are slow, often taking hours to build an object, and the results, while impressive to the eye, can be too crude for extremely closefitting parts.

And as Guslick pointed out, anyone who is desperate for a weapon "has the ability to assemble a zip gun from parts bought in a hardware store for $15."

The National Rifle Association did not respond to messages requesting the group’s position on 3-D manufacturing. But for gun-control advocates, the real worry regarding 3-D printers and other machines is what the future might bring in the way of technological advances.

"Down the road it’s going to be a big concern," said Josh Horwitz, executive director of the Coalition to Stop Gun Violence. "We don’t know how that’s going to come about and don’t know what technology."

Baetzel, for one, said he did not worry about what other people might do with the technology.

"I follow the laws," he said. "I personally think everyone else should follow them."

He said he did not post his designs hoping that someone would use them illegally.

"It was more, ‘Look at this cool thing I did."’