Cali Swag District Video Star Witnessed M-Bone's Murder

We're told a heavyset man who goes by the name, "Bubba" -- was in front of the liquor store in Inglewood, CA Monday night when someone shot and killed M-Bone.

Bubba famously did the "Dougie" without a shirt on in the group's music video -- and is close personal friends with several members of C.S.D.. Several witnesses tell us Bubba was waiting inside of the car M-Bone drove to the store ... and when the fatal shots were fired, shards of broken glass struck Bubba in the face.