Expert explains bump stock likely used by Vegas shooter

Image 1 of 3

Aside from a staggering number of victims -- one thing that set's this week's mass shooting in Vegas apart from others is the way each shot was fired.

We are now learning the gunman had modified at least two of his semi-automatic weapons, basically turning them into a machine guns.

A weapons expert with St. Petersburg police explained what the gunman did and why that has law enforcement officers across the country concerned.

Listening to the gunfire during Sunday night's mass shooting is unsettling, but it is absolutely necessary to understand how the gunman was able to kill so many people, so quickly.

"From an enforcement standpoint it is concerning someone can have that fast of a rate of fire and still be a legal weapon to own," explained Robbie Arkovich with the St. Petersburg Police Department.

The gunman had semi-automatic weapons, where each trigger pull equals one shot.

But when you listen closely, the shots sound more like a machine gun, where the weapon continues to fire as long as the trigger is held down.

Fully automatic weapons are highly regulated but the Associated Press reports the gunman modified his semi-automatic with something called a bump fire stock.

St. Pete police firearms instructor Robbie Arkovich says, bump stocks are legal and easy to buy, while machine guns are not.

"That is a modification that is on the open market and it is easy to do," Arkovich explained.

And that concerns him.

"As a police officer it is concerning because it's a legal modification people can make and if it's a situation like Las Vegas, it can be used against the public."

Arkovich says law enforcement is already learning from Vegas, and adjusting their own training here at home.

Many are now asking about the legality of bump stocks and who can buy them.