3-D printed ‘ghost guns’ pose new challenges for crime-scene investigators

New methods are needed to do ballistics on plastic guns — before the weapons gain popularity

plastic revolver

An assembled plastic Washbear revolver sits inside a 3-D printer at the University of Mississippi in Oxford.

Megan Wolfe/Univ. of Mississippi

On December 17, 2017, police responded to reports of gunshots at a Phoenix apartment. When Cleophus Cooksey Jr. answered the door, his mother and stepfather were lying on the living room floor, shot dead. Police arrested Cooksey.

The double homicide seemed like an isolated incident, a violent end to a family dispute. But ballistics evidence gathered at the crime scene told an even bigger story.

Firearms leave telltale markings on the bullet and the cartridge case that’s ejected when a pistol or rifle is fired. The U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives catalogs these marks in the National Integrated Ballistic Information Network, or NIBIN.

Police had collected cartridge cases from that Phoenix apartment. Within 48 hours, the NIBIN database revealed ballistics matches that linked weapons used in several other murders during the previous three weeks. Cooksey has been charged with killing eight people.

Although not perfect, ballistics evidence helps police pull suspects off the streets. NIBIN has yielded over 110,000 matches since it was launched in 1999. But a new type of gun — made of plastic using 3-D printers — may bring new challenges for forensics experts.

Use of a 3-D printed weapon “would make it very difficult for NIBIN to detect the signature of that weapon,” says Frank Fernandez, a retired police chief based in the Miami area who chairs the firearms committee of the International Association of Chiefs of Police.

Right now, violence involving 3-D printed guns is more of a risk than a reality. The most commonly available 3-D printers, which cost hundreds of dollars, may not print usable guns, and high-end models cost tens of thousands of dollars.

But 3-D printed guns have been confiscated at airport security checkpoints, including a disassembled gun seized July 3 at New York’s LaGuardia Airport. And in February, a Texas man who had been prohibited by a judge from possessing firearms was sentenced to eight years in prison for carrying a hit list and a gun with 3-D printed parts.

As 3-D printers improve and costs come down, some experts worry that more people will decide to print guns. Because knowing how to analyze the evidence 3-D printed guns leave at a crime scene may one day become an important skill, researchers are making and firing plastic guns to figure out the forensics of these DIY weapons.

Not a plaything

At less than a pound, the milk-colored pistol looks and feels like a toy. Its parts are formed from plastic spit out with precision by a 3-D printer. But the device shoots real bullets. With two fingers, analytical chemist Oscar “Beau” Black squeezes the trigger, sending a bullet into a target in a laboratory at the University of Mississippi in Oxford.

After a shooter pulls the trigger on a gun — 3-D printed or conventional — a firing pin sets off an explosive substance, which ignites the gunpowder that’s packaged with the bullet in a cartridge. The resulting pressure pushes the bullet out of the cartridge and through the gun’s barrel. The firing pin leaves a dent on the metal case that holds the cartridge together.

The barrel of a conventional gun typically has spiral “rifling” grooves that spin the bullet to give it more stability during flight. These grooves gouge lines around the circumference of the bullet. The number, angle and direction of the marks may reveal the gun that fired it.

The plastic gun barrels that Black built did not have rifling grooves. But even if a plastic gun did have grooves, the plastic itself would be too soft to dent the bullet.

If a plastic gun were used to commit a crime, tracking down the weapon’s maker and the shooter would not be easy. Unlike conventional firearms, plastic guns have no serial numbers for tracking their source, says James Cizdziel, an analytical chemist at the University of Mississippi, where Black is a postdoctoral fellow. That’s why printed guns are called ghost guns, he says. Plastic guns can also be taken apart and sometimes destroyed by dissolving their parts in solvents, Black adds.

“The reality … is that a 3-D [printed] weapon could potentially circumvent a lot of the [existing] security measures,” including background checks and metal detectors, Fernandez says. “Anyone can make them and [not] have to go through any type of vetting process.”

At the scene of the crime

Conventional metal guns and 3-D printed plastic firearms differ in their makeup and the clues they leave behind.

Conventional metal gun3-D printed plastic gun
Evidence from the gunIf a gun is left at a crime scene, its serial number can be used to find its owner. Metal guns typically don’t leave fragments or shavings when fired.Plastic guns have no serial number. When bullets are fired, plastic gun flakes can be found on bullets and on the ground below a bullet’s trajectory.
Marks on bullets and cartridgesSpiral ridges in the barrel leave distinctive marks on bullets. The firing pin also leaves a mark on the ammunition cartridge case.The barrel leaves no marks on bullets, though the metal firing pin can leave marks on the cartridge.
Gunshot residueChemical residue from the barrel can give clues about the shooting distance.Chemical residue is sprayed from the gun, but its relevance has not been tested.
FingerprintsPrints can be lifted from a metal gun.The texture of gun plastic may obscure prints. Chemicals used in traditional fingerprint-lifting methods might react with a gun’s plastic, making fingerprints unrecoverable.
DNA recovery Swabbing the gun may yield DNA, especially if skin cells, blood or sweat is left on the gun.Textured plastic may make it easier to recover DNA from skin cells, blood or sweat left on the gun.

Sources: O. Black; O. Delémont; Derek Proctor/Tennessee Bureau of Investigation

A poor substitute

To see what kind of clues plastic guns do leave, forensic scientist Olivier Delémont and graduate student Hanna Honsberger of the University of Lausanne in Switzerland and colleagues began printing and testing a weapon known as the Liberator.

The Liberator made a splash in 2013 when Defense Distributed, a 3-D printing and firearms company, released blueprints for the gun, the first made entirely with printable parts except for a metal nail used as the firing pin. Within a couple days of its release, the blueprint was downloaded about 100,000 times. Defense Distributed has since been ensnared in lawsuits with U.S. states trying to keep the group from sharing the blueprints.

The Swiss researchers printed and assembled six Liberators with a plastic called acrylonitrile butadiene styrene and fired them in a test chamber. Radar clocked the speed of the bullet as it exited the gun at between 138 and 172 meters per second, the researchers reported in 2018 in Forensic Science International. The same type of bullet launched from a typical handgun exited at about twice that speed.

The scientists fired a couple of bullets from plastic guns into a block of ballistics soap, the kind used to test a projectile’s potential to wound. The faster of the two bullets lodged 21 centimeters into the block, suggesting that the gun could cause damage similar to a typical handgun of the same caliber.

Most of the plastic guns broke upon firing; the frame cracked or the barrel fell out. On occasion, the weapons also ejected the nail, and bullets often deviated from their intended path. Good thing the scientists used a machine to fire the gun while they watched safely behind bulletproof glass.

Delémont doesn’t think such a gun would tempt anyone who already has access to a regular firearm. Based on the guns his group tested, “it would be more dangerous to be the shooter than to be the target,” he says.

Nevertheless, the Liberators left clues that could help crime scene investigators. Large pieces of plastic, like the barrels that broke away, could reveal the type of printer used, since various printer models produce printed plastics with visibly different textures, Delémont says.

Even if a shooter managed to avoid leaving a chunk of the gun behind, little bits of plastic might give the gun away. After firing, small fragments of plastic peppered the ground below the bullet’s path, the Swiss researchers reported in the February Forensic Science International. The metal bullets and cartridges also picked up near-microscopic bits of plastic. Under a blue forensic light, plastic residues on the spent ammunition popped into view.

bullet analysis
When the bullet at left, which was shot from a plastic Liberator gun, is put under blue fluorescent light, the plastic polymer from the gun becomes visible (right).H. Honsberger et al/Forensic Science International 2019

Chemical traces

Back at the University of Mississippi, the plastic guns Black printed held up better than the Liberators. One weapon even fired several dozen times without any visible wear and tear, he says.

Black and Cizdziel first got into 3-D printing to replicate parts of lab equipment. The two used the knock-offs to show chemists-in-training how expensive equipment worked without having to pull apart a real instrument’s guts.

When 3-D printed guns began making headlines, Black, who learned how to shoot from his father as a kid in Texas, pondered how one would trace a phantom firearm. In 2016, Black and Cizdziel took up the chemistry challenge, which became Black’s doctoral project.

Black has printed and tested several copies of two models — a pistol called the Songbird and a revolver called the Washbear — using several different types of plastic. Though each gun took him dozens of hours of printer time, making the guns was surprisingly easy, Black says.

3-D printed gun testing
The University of Mississippi team sometimes used a machine to pull the trigger on 3-D printed guns, then looked for clues on bullets, cartridge cases and the target.O. Black et al/Forensic Chemistry 2017

“That Washbear revolver is a fully functional five-shot revolver,” he says. The blueprints, which he downloaded from a website, weren’t difficult to find, though they tend to be taken down just a few days after posting.

Like Delémont’s group, Black and Cizdziel found plastic on bullets and cartridges after the guns were fired.

To see if the shreds of plastic could be matched to their starting material, Black and Cizdziel turned to a technique called direct analysis in real-time mass spectrometry, or DART-MS for short. In forensics, DART-MS has helped investigators analyze seized drugs and explosive residues left on fingerprints.

The DART-MS spews a hot stream of gas molecules into a sample, in this case, a bit of plastic scraped from a bullet. The stream heats up chemicals in the plastic, which react with the gas and get sucked into a mass spectrometer. The instrument sorts the sample’s molecules by mass. The process takes only seconds to yield a chemical fingerprint for all compounds that the instrument finds.

Black and Cizdziel were able to match the plastic on the bullets and on the T-shirt they used as a target to the original plastic used in the printer. The process worked even with the barrel-scraping friction and heat of being fired. The pair described the work in 2017 in Forensic Chemistry and has started more tests on a wider variety of plastics.

Plastics for 3-D printing can contain dozens of additives, such as dyes, that give them different properties. This provides chemists with an opportunity to nail down the specific source of the plastic used to make a gun.

Black and Cizdziel are building a database to catalog potential sources of plastics. So far, the pair has analyzed 84 of the hundreds of plastics that Black estimates are available for gun-making. The idea is that if law enforcement needs to trace a plastic, investigators might find the plastic’s match in the database and possibly track it down to a handful of stores.

The pair is also studying whether traditional crime scene investigation tactics can be applied to plastic guns: Can fingerprints be lifted from the textured plastic? How well do the guns hang onto DNA from the shooter?

Doing this work now makes sense to retired police chief Fernandez, who expects forensics investigators will eventually be contending with 3-D printed guns. “With technology today,” he says, “it’s just a matter of time before [people] figure out how to make a functioning multishot handgun work consistently.”

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