They open fire toward the Gulf of Mexico below.
The whole thing was videotaped, edited and posted to the YouTube channel, Do It With Dan.
"What are the legalities of this," asked Dan, the host, as they walk toward a single-engine plane at Peter O. Knight airport for their flight.
Tampa gun maker Mark Serbu, 55, holding a camouflage backpack with the gun barrel sticking out, answers, "I didn't really ask. I'm assuming it's okay."
The Tampa Police Department agreed with Serbu on Monday, but only after taking the matter under advisement. Law enforcement agencies were alerted to the video by the Hillsborough County Aviation Authority, owners and operators of Peter O. Knight.
The Federal Aviation Administration also is investigating, but would not elaborate Monday.
A lawyer specializing in aviation issues, Guy Haggard of Orlando, said it wouldn't surprise him if the FAA follows the lead of Tampa police.
"People use helicopters for things like hog hunts," Haggard said. "In Florida, you can shoot guns from the sky in some situations."
And under FAA regulations, Haggard said, it is legal to drop an object from an aircraft as long as reasonable precautions are taken to prohibit injuries.
Bullets could be defended as "dropping objects," Haggard said.
The YouTube video includes a slide that says the trio flew for hours to a "safe location" before pulling out what they call a "machine gun." Serbu, the pilot and the owner of Serbu Firearms in Town 'N Country, identified the gun to the Tampa Bay Times as a fully automatic, Heckler & Koch MP5 9 mm submachine gun.
Since no one called law enforcement to complain about gunfire, it appears no one was put at risk, said Tampa police spokesman Stephen Hegarty.
Serbu told the Times he circled the Gulf of Mexico between Clearwater and Tarpon Springs for more than an hour before settling on a safe spot a couple miles from shore at an altitude of about 3,000 feet.
Until the Times contacted him Monday, Serbu said he didn't know the video had drawn the attention of authorities.
"There was no one close to us, no one for at least 10 miles," he said.
"We did everything safe. I had my daughter with us."
Valerie Serbu, 22, is the third person on the plane.
The trip was the idea of the show's host — a man identified on YouTube as Dan who says he's 24 and from Atlanta but wouldn't give the Times his last name. His channel specializes in videos of motorcycle and firearms stunts and has over 800,000 subscribers.
"He just thought it would be fun to shoot guns into the water from a plane," Serbu said with a chuckle.
Some of Dan's viewers had negative things to say in the YouTube comments section, so he took the video down last week — but not before someone copied it to send law enforcement. Dan put the video back online Monday afternoon.
Neither Serbu nor Dan could remember the date of the flight. Police learned from personnel at Peter O. Knight that they flew out of the Davis Islands airport Feb. 21.
In 2013, Serbu made national headlines by refusing to sell semi-automatic, .50-caliber rifles to the New York City Police Department because he disagreed with the NY SAFE ACT that classifies the weapon as an assault rifle, banning it from use by the public.
Serbu planned on selling shirts with the image of the Soup Nazi from the Seinfeld sitcom and the words, "Hey NY-Gov ... No Serb For You" — a play on the character's "No soup for you" catchphrase.
Actor Larry Thomas, who portrayed the Soup Nazi, demanded his likeness be removed so Serbu replaced it with one of him.
"Looks like I did it again," Serbu said with another chuckle Monday.
Near the end of the video, host Dan says shooting a gun out of a plane "might be the most American thing" he's ever done.
The video concludes with a teaser for another of his YouTube feats — firing off flamethrowers in his front yard.