OLD TOWN, Maine — Mack Gwinn III doesn’t like advertising his company’s whereabouts.

But he loves advertising his product, which he calls the most versatile assault rifle on the planet.

Mack Gwinn Industries, or MGI, is tucked into an unassuming, homey building on Main Street. The business isn’t open to the public and the doors are locked even when employees are inside.

“We try to maintain a low profile,” Gwinn said Tuesday, adding that his company has a close relationship with Old Town police because it’s important to keep MGI’s product out of the wrong hands.

MGI designs and manufactures the Hydra modular weapons system, which has a similar look and feel to the AR-15. But Gwinn’s version of the assault rifle allows the operator to change the barrel, magazine well and other parts at will to fire more than 30 different calibers of ammunition.

Doing that sort of alteration on an AR-15 is like changing the engine in a car, Gwinn said. On a Hydra, the switch doesn’t require any tools and can be finished in less than two minutes.

The record time for a full caliber switch by an MGI employee is 34 seconds, according to Gwinn.

The Hydra name came from Charlie Cutshaw, a journalist who, because of the gun’s ability to fire just about any round, compared the weapon to the mythical many-headed serpent. The nickname stuck.

“The weapon system is capable of firing anything from a .22-caliber bullet all the way through a .50 Beowulf,” Gwinn, a Bangor High School graduate, said as he slid a different barrel in place while demonstrating how simple it is to change the caliber of the gun.

Gwinn also showed how to swap out the Hydra’s magazine well, which allowed him to load a magazine from one of the most widely recognized weapons on the planet — an AK-47.

Using a magazine and magazine well specifically designed for the type of ammunition the shooter is using is always more reliable than trying to force ammo through a mismatched piece of equipment, he said.

MGI even manufactures an attachment that allows the Hydra to fire a belt of ammunition.

The company markets the Hydra to personal gun enthusiasts, law enforcement and the U.S. military. Some police officers in Bangor, Milo and Machias, and members of the Penobscot County Sheriff’s Department, carry the assault rifle in their trunks, Gwinn said. MGI products are available for purchase online and also can be found at Maine Military Supply in Holden.

The Hydra lets officers drop an ammunition magazine out of their sidearm and load it into the assault rifle, allowing them to keep a larger distance between themselves and an armed suspect.

“That’s my idea of a fair fight for law enforcement professionals,” Gwinn said.

The standard weapon system retails for about $1,200, not including additional barrel and magazine well attachments.

Demand for the Hydra is high, according to Gwinn. In January alone, employees from the 12-person company attended five trade shows, which led to more than $250,000 in sales, he said.

“I love starting the year that way, but it makes forecasting sales difficult,” Gwinn said.

MGI became incorporated in 2005 but has more than 40-year-old roots in Maine.

Gwinn’s father, Mack Gwinn Jr., founded Gwinn Firearms after his return from the Vietnam War. In the early 1970s, Gwinn Firearms became Bushmaster, which evolved into one of the largest firearms companies in the country.

Gwinn Jr., who designed several pistols, machine guns and magazines, is “mostly retired” from the industry now, according to his son.

Both Gwinns are special forces veterans.

Gwinn III described a gun as nothing more than a tool that allows people to hunt, protect themselves or enjoy a day at the shooting range.

Gwinn said people sometimes grill him about his product, arguing that a “tool” doesn’t kill people.

“But any knife or screwdriver can kill somebody if it’s misused,” Gwinn said, adding that a proper education about weapon safety and respect for a weapon’s power are extremely important for any gun user.

Who does Gwinn think should own a Hydra?

“In my opinion, just about everyone,” he said.