Blac Youngsta is behind bars in Houston after cops say the rapper was carrying prohibited ammo when they pulled over a car in which he was a passenger.

Law enforcement sources tell TMZ ... Youngsta and 3 friends were driving around H-Town Wednesday afternoon in a GMC Yukon -- with BY in the backseat -- when cops pulled 'em over for running a red light. We're told the officers smelled marijuana, so they searched the car and found 3 pistols and weed.

According to the criminal complaint, Youngsta is charged with felony possession of a prohibited weapon -- specifically, armor-piercing ammunition. It's unclear if the ammo was loaded in a handgun or separate.

We're told another passenger, Marcus Dion Cartwright, was charged with unlawfully carrying a weapon. The 2 other people in the vehicle were NOT charged.

Blac Youngsta is currently locked up in Harris County Jail.

(Story Via TMZ)