'Southland' Star -- Gun Drama During LAPD Ride-Along

"Southland" star Michael Cudlitz was riding shotgun with the LAPD during a police ride-along this weekend ... when suddenly, a call came in about a "man with a gun" ... and Cudlitz's car raced to the scene.

Law enforcement sources tell TMZ ... the piece turned out to be an "air soft" gun, which looked like a real firearm, but shoots plastic pellets instead of real bullets. The man in question was not arrested.

The gun drama was just one of several calls Cudlitz's car responded to that night ... while Michael was doing research for his role as a cop on the TNT show.

We're told Cudlitz got some great real-life experience as his unit patrolled Hollywood Blvd. ... while the cops made sure the actor was never in harm's way.

Sources tell us Cudlitz impressed everyone by asking a ton of questions and even puling a double shift -- 12 p.m. until 3 a.m.