Need Advice Quick: Irresponsible minor in my apartment complex has an illegal gun.

Kid is 17, he somehow was able to acquire a 6 shooter revolver, with lots of bullets. Red Flags immediately popping up everywhere when the kid took it out of the gun case fully loaded and started waving it around with his finger on the trigger. Very afraid of accidentally discharge going off and killing someone.

Should I talk to his parents about this or just inform the cops and feel semi guilty I helped but someone in jail for a really long time since my state and county have zero tolerance policies regarding guns.

*Edit: My Room mate eventually called the cops because he freaked out to bad about it. Problem is I was the one who saw him with the gun not my room mate. Now my room mate wants me to tell the cops specifically what I saw.

I'll keep updating as a I go along.

Edit 2: cops with shot guns quietly came around and walked around , they said there wasn't much they can do and left. WHAT THE FUCK