Computer files to create a handgun almost entirely from parts made with a 3D printer went online Monday, alarming gun control advocates after it was successfully test-fired by its inventor.

The single-shot .380-caliber Liberator bears a vague resemblance to its namesake, the FP-45 Liberator pistol that the United States developed during World War II to be air-dropped to French Resistance fighters.

Computer-aided design (CAD) files for the Liberator appeared on the website of Defense Distributed, a non-profit group that promotes the open-source development of firearms using 3D printers.

"We'll build the trigger first... Next, we'll build the hammer subassembly ... Next, drop the hammer into the frame...," reads the accompanying set of instructions, which come in English and Chinese.

"Finally slide the grip on the frame and insert the grip pin. Your Liberator is now ready to go!"

For the Liberator to conform with US firearms law, the instructions call for an inch-big chunk of steel to be sealed with epoxy glue in front of the trigger guard, so that the weapon can be spotted by metal detectors.

The only other non-plastic part is a tiny nail that acts as the firing pin.

Business magazine Forbes posted a video of the Liberator being remotely test-fired outside Austin, Texas last week, with a yellow string tied to the trigger of the toy-like white-and-blue handgun.

"The verdict: it worked," Forbes reported, adding however that the Liberator exploded ("sending shards of white ABS plastic flying into the weeds") when its inventor Cody Wilson attempted a second test using a rifle cartridge.

"I feel no sense of achievement," the 25-year-old University of Texas law student told Forbes. "There's a lot of work to be done."

CAD files for gun parts have been available on the Internet for some time, but the Liberator is apparently the first entire weapon ever to be fabricated almost exclusively with parts created with 3D printing technology.

Supporters of tougher gun laws in the United States -- where there are nearly as many guns (an estimated 300 million) as there are people (about 315 million) and more than 30,000 gun-related deaths a year -- expressed alarm.

"Stomach-churning," said Senator Charles Schumer of New York. "Now anyone -- a terrorist, someone who is mentally ill, a spousal abuser, a felon -- can essentially open a gun factory in their garage. It must be stopped."

In the House of Representatives, Congressman Steve Israel, also from New York, is sponsoring an Undetectable Firearms Modernization Act to outlaw plastic homemade guns.

"Security checkpoints, background checks, and gun regulations will do little good if criminals can print plastic firearms at home and bring those firearms through metal detectors with no one the wiser," he said in a statement.

No longer prohibitively expensive, 3D printers can now be bought for about the same price as a top-end laptop computer. Brooklyn-based MakerBot, for instance, markets its desktop Replicator 2 for $2,199 with delivery in a week.

After the December 2012 massacre of 20 children at Sandy Hook elementary school in Newtown, Connecticut, MakerBot took down CAD files for semi-automatic rifle parts that gun enthusiasts had posted on its open-source 3D printing library.