Ex-UFC Star Chris Leben -- 30 Days In Jail ... For Assault Rifle Incident

Chris Leben -- former UFC fighter who appeared on Season 1 of "The Ultimate Fighter" -- will do 30 days behind bars for bringing an ASSAULT RIFLE to his estranged wife's apartment building.

The 35-year-old was arrested on June 8th after his estranged wife called 911 and said Leben was trying to scale a wall and break into her apartment with a loaded .45 caliber handgun.

Officials say he was also in possession of a AR-15 assault rifle and was wearing a shirt that said, "Shoot First."

Leben was eventually charged with a slew of crimes -- including felony possession of an assault weapon, violating a court order and several counts of vandalism.