HARLINGEN – The frightening sounds of rapid fire ringing out were captured by the cell phones of Las Vegas concertgoers.

Some of the rounds raining down from the 32nd floor of the Mandalay Bay Resort carried a distinct pattern.

Rio Grande Valley gun store owner Brian Guerra tells CHANNEL 5 NEWS some of the sounds ringing out on the strip were consistent with a fully automatic machine gun.

"The spacing between each shot and everything sounded like something that could be done automatically," said Guerra.

There is only a handful of ways to obtain a fully automatic weapon. The only legal means is purchasing a weapon built before 1986 and registered through the National Firearms Act.

Other methods include converting a semi-automatic weapon to fully automatic through the use of conversion kits.

Kits refer to gun parts that allow semi-automatic weapons to cross the threshold into full automatic. Owning the parts isn't necessarily illegal. Altering the gun to fit the parts is.

"The parts that make a gun fully automatic could be purchased,” said Guerra. "But once they're put into the firearm that now becomes an illegal item."

We had Guerra listen to the gun sounds from the scene that had a more inconsistent pattern. He said these are characteristics of semi-automatic weapons with modifications.

These modifications allow semi-auto weapons to simulate a fully automatic weapon.