Laboratory test ???

[ruger44srh]

What "laboratory test" can show a person fired a gun "at least 13 times" ?? Just saw this on the morning news and made me wonder.

 

[17 squirrel]

I would guess, gun shot residue patterns in multiple places.
Say the shooter was walking down a hallway firing a pistol, there would be patterns on the walls from each time the pistol was fired.

 

[Woobie]

It's probably their fancy way of saying they found 13 cases on the ground. They can probably confirm the primers were struck by the same pin in a lab.