Watch NBC's Ken Dilanian break down what makes 'bump fire stocks' so deadly

In a heartbreaking report with MSNBC's Joy Reid, reporter Ken Dilanian broke down talking about the dangerous modifications known as "bump fire stocks" that make semi-automatic weapons into terrifying, mass killing machines.

Experts told Dilanian that without the addition of the bump fire stock, Stephen Paddock would not have been capable of killing as many people. "These allowed him to just pour fire down on the crowd," he said.

He also noted that he hasn't heard gunfire like that since he was embedded with the U.S. forces in Iraq. "That sounded like a war zone," he said of the released tapes of the Las Vegas massacre.

Watch in the video below:

New: ATF says Paddock had 12 bump fire stocks -- legal modifications that allowed him to pour fire on the crowd https://t.co/HcYA4UTwA5

— Ken Dilanian (@KenDilanianNBC) October 4, 2017