The rapper -- who hails from the Louisville area -- was partying at a place in town called Vibes Restaurant and Ultra Lounge ... which was throwing a shindig in honor of the Kentucky Derby, something Jack himself had attended earlier in the day Friday.

Later that night, he stopped in at Ultra Lounge ... where he was filmed vibing out near the stage. Check out the video though -- it looks like an altercation unfolds with a woman and at least one man ... who then start leaning into each other before falling over in a scuffle.

Right at that moment, you hear what sounds to be a gunshot go off ... and everyone starts making a run for it, including Jack. It doesn't appear he was injured -- but we got calls in to his rep to make sure.

While it's unclear what exactly led up to the violence or who shot ... it sadly ended with someone losing their life. Police have said a woman was found dead from the shooting and a man was transported to the hospital with non-life-threatening injuries.