In the heart of Holly Hill’s manufacturing district, a stone's throw from the railroad tracks, a small white sign on a warehouse wall identifies Elite Custom Railing.

But as manager Amanda Black will tell you, the company doesn’t make railings anymore.

Instead, stacks of boxes of gun parts can be seen from the street through the open warehouse door, each box filled with semi-automatic rifle kits ready to be shipped throughout the country.

Though it sells many gun parts, the shop’s specialty is the “unfinished lower receiver,” which houses the trigger mechanisms and other parts that when combined make a workable gun.

It’s the key part in kits that allow the buyers to build a do-it-yourself AR-15, a military-style rifle, a version of which was used in the 2012 Sandy Hook Elementary School mass shooting in Newtown, Connecticut.

The kits can be more expensive and are certainly more time-consuming than buying a rifle off a store shelf, but they come with features the finished product can’t match.

The parts have no markings — no manufacturer’s stamp or serial number — making them effectively untraceable. These so-called “ghost guns” can be purchased without a background check and no mandatory three-day wait.

And ghost guns are becoming increasingly popular.

“We were doing aluminum railings for years,” Black said recently as she juggled orders at her office desk.

But making untraceable gun parts is better business. It keeps the 15-person shop at Elite Custom Railings pretty busy.

“Too busy," Black said, letting loose a sigh of exhaustion. She estimates the small shipping location sells between 100 and 150 unfinished lower receivers a day.

On the low end, that’s potentially 26,000 AR-15s a year from a single shop.

There are at least six in Volusia County, which has become something of a hub for the shops.

The kits are legal because of a kink in the way the federal Bureau of Alcohol Tobacco Firearms and Explosives defines a firearm. Manufacturers say many of their buyers are hobbyists or hardcore Second Amendment supporters who want to keep their firearms off the government radar.

But some law enforcement officials worry that the unmarked, unregistered guns could make it easy for criminals to build an arsenal of untraceable, high-powered weapons. And the practice — which manufacturers’ websites promote as easy and quick — circumvents state or federal efforts to shore up gun laws.

"It's legal, but it's almost like a loophole in the law," said Port Orange Police Chief Thomas Grimaldi.

The police chief said he respects a gun owner's wish to keep ownership private, but he worries about those protections going too far.

"We're making it easy for the criminals," he added. "I have a concern — a huge concern over that."

AGE OLD HOBBY

While guns sold in the U.S. are required to have a serial number stamped by the manufacturer, you don't need to have one if you're manufacturing a gun for your own personal use — legal under the Gun Control Act of 1968.

Gunsmithing is older than the Constitution, and for centuries the manufacture of a firearm was something handed down through generations of craftsmen.

Today, parts for a semi-automatic rifle like an AR-15 or Glock-type 9-millimeter handgun can be purchased from one of thousands of websites. The training needed to build the weapon is just a few clicks away, and a plethora of YouTube videos and websites claim to teach the practice step-by-step. Using a kit to build a gun at home can take between two and four hours for someone who knows what they’re doing. A tabletop machine can carve out the necessary bits in an hour.

The key part is the lower receiver in the AR-15 rifle or the frame on a handgun. The ATF tightly regulates construction of those parts.

At Buck's Gun Rack on West International Speedway Boulevard, owner Scott Buckwald picked out an AR-15 lower receiver inside a display case.

"See that?" he said, pointing to a black, roughly rectangular hunk of metal. “It may not look like a gun, but according to federal law, that's a gun. If you take that piece of metal across the border into California you have to register it with a federal firearms license dealer.”

Buckwald’s receiver is regulated because it’s been completely drilled out, so it’s ready to lock in the other parts of the gun. The ATF sets the definition of a finished receiver as one that is more than 80 percent complete. The trick behind a "ghost gun" is to leave the metal only partially drilled — commonly referred to as an unfinished receiver, a blank, or an 80 percent receiver.

"It's not considered a gun," said Buckwald, who does not deal in unfinished lower receivers. "It's just a chunk of aluminum."

And chunks of aluminum aren't tracked by the government.

INDUSTRY GROWTH

On the outskirts of Port Orange, at the corner of Tomoka Farms and Taylor roads, the Country Garden Feed store sells farm and garden supplies as well as livestock. It was a natural evolution to move into selling survivalist gear, like hunting knives and generators. Now owner Michael Glasnak also sells gun parts online.

Sales are good; while the other local gun-parts sellers offer AR-15 lowers in packs of five, Glasnak sells them in packs of 100.

"It's an accessory. We're a prepper store," Glasnak said, referring to survivalists who stockpile food, ammunition and other supplies in preparation for a possible disaster.

Glasnak said he's been selling 80-percent lowers for about a year, and after a few months of ironing out details for his online business, sales really took off this spring. The husband-and-wife team have sold more than 1,000 unfinished lower receivers since then, they said.

“We’re getting to where it’s going to be the main business right now,” said Becky Glasnak.

"Colorado, California, Michigan, Illinois, New York, Florida…” Becky Glasnak went on to rattle off a list of states on her shipping labels — at least 12 — and those were just from the previous day’s sales slips.

Michael Glasnak said it’s not up to him to vet his customers, most of whom he assumes are survivalists preparing for some kind of dystopian future.

Asked how they can be sure they aren't selling to someone who isn't legally permitted to own a weapon, the Glasnaks say, "We can't."

"People who are making their own guns like this are preppers and they're not gonna let them get into somebody else's hands," he said.

Jim Jusick, co-owner of Tactical Machining in DeLand, insists any criminal who wants a gun would rather steal one than build it himself.

He mentions 20-year-old Adam Lanza, who killed 20 students and six adults at Sandy Hook. His AR-15-style Bushmaster .223 rifle wasn’t homemade.

“He murdered his mother to take her gun,” said Jusick, whose business manufactures both guns and parts, with an online component that’s a hugely popular site for unfinished lowers.

Jusick’s point: Why buy a part of a gun from him when you can steal one that’s already put together?

“That’s a hell of a lot easier than buying an AR-15 (lower), buying the rest of the parts to build it and then figure out how to build it. Figure out how to machine it, machine it, make it actually work, putting it together, test fire it to make sure it runs. Go through all that – okay, now I’ve got a gun. That took me three weeks.

"Put it like this," he added. "There is a possibility — just like getting struck by lightning on a cloudy day. But Joe Terrorist, he's not a builder."

LAW ENFORCEMENT REACTION

Some in law enforcement believe it’s naïve to assume homemade guns don’t fall into the wrong hands.

On a June morning in 2013, John Zawahri shot and killed his father and brother, burned down his family’s home, carjacked a vehicle and forced the driver to take him across town to Santa Monica College, police reports show. There, he killed three more people before he was fatally shot by officers. Zawahri used an AR-15-style rifle, which he’d put together himself from legally purchased components.

In July, Baltimore police said Dayton Harper used a homemade AR-15 rifle to open fire on four police officers, who had responded to reports of shots fired. Harper was fatally shot after police said he fired upon the plainclothes officers.

More homemade guns are popping up, especially in California. Special Agent Graham Barlowe of the ATF’s Sacramento field office said he’s seen a surge of unserialized guns showing up at his crime scenes, with several hundred unmarked weapons confiscated in the past few years.

"Some single transactions will be 50 at a time," he said.

In July, California Gov. Jerry Brown signed a law requiring anyone planning to build a homemade firearm to first obtain a serial number for the weapon and submit to a background check. Gun-rights advocates say the effort will have little effect on criminals and merely serves to make criminals out of law-abiding hobbyists.

Ghost guns haven’t been as prevalent in Florida, said ATF Tampa Special Agent Kevin Richardson — who said he learned of the trend only after watching a National Geographic special on the subject in January.

But there’s really no way to know how common ghost guns are in Florida — because several law enforcement agencies here say they don’t have a uniform method for tracking them at crime scenes.

If police find a weapon at the scene of a crime, they usually make note of the make, model, serial number, caliber and type of weapon in the crime report. If the serial number has been obliterated, they note that, too. But if the gun has no marks at all, local police say, there’s generally no standard way to note that.

The ATF’s Barlowe said the push to use a standard format has been difficult.

"We've been doing outreach since 2013,” he said. But word doesn’t seem to have traveled from California.

Barlowe said local law enforcement officials may be fooling themselves if they think ghost guns aren’t going to prove to be a problem in Florida, too.

“I don’t want to say that I know that Florida has this situation going on,” Barlowe said. “But when we look at the sales that have been going on — when we look at the people that we have been involved with here in California — they’re selling on the Internet to people all over the country. And I just can’t imagine that there would be an exception in Florida.”