According to court records, a grand jury failed to indict Z-RO Tuesday on felony assault charges. TMZ broke the story ... Z-RO's ex-gf claimed the rapper slapped, punched and dragged her around his house while holding a pistol.

Brittany even handed over an audio recording of the alleged beating, prompting cops to arrest Z-RO in July.

Brittany later told us -- on "Raq Rants" -- Z-RO rapped about the alleged incident, and claimed he had "no soul in his eyes" during the attack.

Z-RO's attorney, Charles Adams, tells TMZ ... there were huge inconsistencies between the recording and the written statement Brittany provided cops. He believes that's ultimately why the grand jury exonerated Z-RO.