As 3-D printed guns have evolved over the past 18 months from a science-fictional experiment into a subculture, they've faced a fundamental limitation: Cheap plastic isn't the best material to contain an explosive blast. Now an amateur gunsmith has instead found a way to transfer that stress to a component that's actually made of metal—the ammunition.

Michael Crumling, a 25-year-old machinist from York, Pennsylvania, has developed a round designed specifically to be fired from 3-D printed guns. His ammunition uses a thicker steel shell with a lead bullet inserted an inch inside, deep enough that the shell can contain the explosion of the round's gunpowder instead of transferring that force to the plastic body or barrel of the gun. Crumling says that allows a home-printed firearm made from even the cheapest materials to be fired again and again without cracking or deformation. And while his design isn't easily replicated because the rounds must be individually machined for now, it may represent another step towards durable, practical, printed guns—even semi-automatic ones.

The 3-D printable blueprint of Michael Crumling's test gun, with his specially designed ammunition round shown in darker grey. Michael Crumling

"It’s a really simple concept: It’s kind of a barrel integrated into the shell, so to speak," says Crumling. "Basically it removes all the stresses and pressures from the 3-D printed parts. You should be able to fire an unlimited number of shots through the gun without replacing any parts other than the shell."

Last week, for instance, Crumling shot 19 rounds from a 3-D printed gun of his own design created on an ultra-cheap $400 Printrbot printer using PLA plastic. (He concedes his gun isn't completely 3-D printed; it uses some metal screws and a AR-15 trigger and firing hammer that he bought online for a total of $30. But he argues none of those parts affected the gun's firing durability.) Though the gun misfired a few times, it didn't suffer from any noticeable internal damage after all of those explosions. Here's a time lapse video that shows 18 of those shots.

When the top of Crumling's gun shattered in an earlier test, he determined that the breakage was caused by the shell's movement, not the explosion inside of it. So in his most recent gun design, the roof of his gun's chamber is left open. That allows the shell to eject itself, as shown in the GIF below.

A slow-motion capture of Crumling's gun firing his specially designed ammunition. Michael Crumling

Combined with revolver components or some sort of auto-loading mechanism, Crumling believes his method could enable printing a semi-automatic weapon. "That’s the main reason I developed these, and that’s the next step," says Crumling. "This is a building block for the future of 3-D printed firearms that will enable people to develop semiautomatic and—if you had the proper legal paperwork—even fully automatic weapons."

In the meantime, Crumling's ammunition could demonstrate a controversial new upgrade to the durability of even single-shot printed guns. Law enforcement bodies around the world have responded to the threat of 3-D printed weapons by noting their unreliability. The US Bureau of Alcohol Tobacco and Firearms released a video last year showing a Liberator, the first 3-D printed weapon created by the libertarian group Defense Distributed, exploding during ATF's test firing. Australia's New South Wales police commissioner Andrew Scipione held a press conference last year to warn about the guns' danger after blowing up a Liberator of its own. "No matter what end of this gun you can be on, you could die," he warned.

Defense Distributed has responded by arguing in each case that government officials used the wrong printing methods or the wrong caliber ammunition, perhaps intending to scare gun enthusiasts away from printable weapons. But Defense Distributed engineer John Sullivan also admits the Liberator and other plastic-barrel guns take significant damage every time they fire a conventional round. Because typical ammunition uses thin brass casing, the metal deforms and transfers the round's explosion to the gun's plastic pieces. "That's why a printed barrel works for only say, 10 rounds," Sullivan says. "It just wears out."

For now, the limiting factor to Crumling's rounds may be the difficulty of producing them. So far he's machined the ammunition, which he's named the .314 Atlas based on its .314-inch caliber and the 1920s Atlas lathe he uses to make it, shell by shell. The process takes a painstaking 60 minutes per round. But the raw materials cost only 27 cents each, he says. And once the shells are produced they can be fired, then repacked with new bullets, gunpowder and primer to be shot again.

The chamber and barrel of Crumling's gun shows no visible damage after firing 19 of his .314 Atlas rounds. Michael Crumling

Crumling's steel-shelled rounds seem to control their explosions well enough to protect printed guns created with even the very cheapest printing techniques. "This guy has refined 3D printed firearms such that they can be reliably printed on very low end 3-D printers," says Sullivan. "It’s so brilliantly simple. I love it."

Despite the gun control firestorm associated with 3-D printed weapons, Crumling says he's not particularly interested in his invention as a political provocation as much as an engineering accomplishment he wanted to share with the gun community. "I'm not an activist. I’m more of a challenge-oriented person," he says. "This posed a challenge and that’s much more interesting to me than any political motivation."

That hasn't prevented him from taking some legal precautions. His test weapon contains a chunk of metal to make it legal under the Undetectable Firearms Act and has a rifled bore to comply with the National Firearms Act. Manufacturing your own ammunition is legal in the U.S., though selling it requires a Federal Firearms License. Crumling says he doesn't plan to sell his Atlas rounds, though he's sharing designs for the ammunition on his website. He adds that he could change his mind if enough gun enthusiasts ask to buy the rounds, in which case he'll apply for the necessary license.

Until then, Crumling's 3-D printing-friendly ammo will serve as a proof-of-concept—and a reminder: If gun control advocates are taking comfort in printed weapons' impracticality, that comfort gets a little colder with every upgrade.