Captain Bob Rufo, of the Woburn Police Department, lays out multiple rifles for a weapon demonstration in the basement firing range.
"This is a 30-round magazine," Rufo says, loading a clip. "I'll fire five rounds, semi-automatic." And he fires. Pop...pop...pop...
With a semi-automatic weapon, a round gets fired every time you press the trigger. Even pulling the trigger as fast as you can, the rate of fire is limited.
But as Rufo shows us -- an automatic weapon fires continuously with one pull on the trigger.
Sources tell NBC news the accused gunman in the Las Vegas massacre bought two devices called "bump stocks." They're made to look just like a regular gun stock but convert a semi-automatic weapon into -- essentially -- a fully automatic weapon. The device uses the gun's recoil to rapidly bump the trigger causing it to fire off rounds in quick succession.
"The way it's marketed is 'enjoy it, have fun on the range, feel what it's like to fire an automatic weapon,'" says Rufo. "But in the hands of someone sinister, it's a fully automatic weapon."
He displays a semi-automatic gun with a bump stock. It's startling how quickly Rufo empties the clip. The bump stock can increase the firing rate to 480 rounds-per-minute.
Firing from his hotel window down into the helpless crowd below, the Las Vegas shooter was able to kill 58 people, injuring more than 500 others.
We ask Rufo how long he thought the shooter had been firing to commit that kind of carnage. "Not long," he said. "I would say minutes."
While automatic weapons are only legal for Massachusetts gun owners who are federally vetted collectors or special training officers, anyone in Massachusetts with a license to carry can get a semi-automatic weapon. Bump stocks are legal to buy nationwide, are easy to get, and cost about $100.
The shooter also had tripods set up in his hotel room — possibly, Rufo says, to help him shuttle from rifle to rifle firing off rounds without risking any one rifle from overheating and malfunctioning.
"His intent was to hurt a lot of people. He was prepared," said Rufo. "Very diabolical."