ATF spokeswoman Ginger Colbrun said the agency keeps track of machine-gun owners, but the exact number of civilians in the Houston region who own them was not readily available.
"There are lots of opinions out there on firearms. We are not here to weigh in on any of them," Colbrun said, "just to enforce the laws on the books."
Requirements are about the same as buying a regular gun. Owners generally cannot be felons, must be a U.S. citizen or legally residing in the United States and must promise to keep the gun properly secured.
They also have to provide to authorities the exact address of the guns' owner and be amenable to in-person visits by inspectors. The six-month waiting period is also seen as a possible deterrent to criminals.
A Department of Justice-issued application states that the "chief law-enforcement officer" in the community where an individual gun owner lives must sign a form to confirm he or she has been advised the gun will be in their area and that they have no objection.
Authorities have witnessed the wrath of machine guns in the wrong hands. In an especially infamous 1997 attempt to rob a bank in North Hollywood, Calif., two men in full body armor sprayed bullets at police while using illegal machine guns.
In 2005, five machine guns were stolen from an unlocked vehicle belonging to a gun dealer in The Woodlands who had been at the shooting range and left them when he parked in front of his home. They were later found.
Houston Armory founder Bill Pace said he and other dealers were outraged by the carelessness.
So much firepower in one specialty weapons shop might entice criminals, but the weapons are kept in vaults within a large bank-style vault, protected by security cameras and alarms. Pace's employees are armed and trained, and the police say they can respond to any alarm from the business in less than a minute.
"Any sane person would not try it," he said of potential attackers.
He added, "We observe everyone, and if we don't feel comfortable with the transaction, we won't do business with them."