LENEXA, Kan. -- Police reportedly found 23 firearms in Stephen Paddock's room and 19 others at this home. They believe he had thousands of rounds of ammunition, but which weapons he used to commit the largest mass shooting in modern U.S. history is still being investigated.
Investigators confirmed Paddock used a bump stock -- a modification used to turn a semi-automatic rifle into a rapid-fire weapon.
FOX 4's Shannon O'Brien was in Lenexa after speaking to a gun expert about the types of weapons used in the music festival massacre.
It was the sound of terror as Stephen Paddock stood in his room on the 32nd floor of the Mandalay Bay and showered the crowd below with bullets, killing 59 people and injuring more than 500.
Although gun expert Don Pind does not know for sure what kind of gun was used in the attack, he bases his theory on the sound of the gunfire.
"It sounded like a .308 machine gun," Pind said, displaying two different guns and several types of ammunition. "This is a .308 caliber; this is a .223."
Pind presented two types of guns which are semi-automatic, meaning you must pull the trigger each time a bullet is fired. Depending upon the gun, they can shoot 400 to 750 rounds per minute.
"Here you can fire this gun, 1,2,3,4,5,6,7,8,9,10,11, but its not something that gets very rhythmic. The machine guns will fire fast enough that they will fire a lot faster than you work your finger. So that's the reason most of us were thinking it was some kind of machine gun. Now whether it was commercially manufactured or whether a good gunsmith got ahold of it and did something illegal," Pind said.
Because of the rhythmic sound of the gunfire, Pind believes the guns used by Paddock were modified so they would shoot like an automatic weapon, or machine gun as he calls it. Pull on the trigger once and bullets fire until the trigger is released, or the clip is empty.
"So right now in the United States for you to buy a machine gun it has to be made prior to 1986 and they keep the list of serial numbers on what year they were made," Pind said. "Thats the problem we have, can these be converted? Sure, they can, but none of us want to spend the next 10 to 20 years in Leavenworth."
When asked to display how these guns work, Pind said he could not.
"Our range won't handle the velocity. This range, we have six bays up, six bays down and set up for about 2,000 feet a second. These are ranging a lot higher than this. They will punch holes in the backstop so we don't shoot them here."
Two pictures are circulating around of two of the guns found in Paddock's hotel room. They look similar to the ones Pind showed us. In a news conference Tuesday, police said they are trying to figure out who leaked the photos to the media.