Young Jeezy -- AK-47 Wasn't His ... It Belongs to Concert Security Chief

The AK-47 that got Young Jeezy and 5 others thrown in jail doesn't even belong to the rapper -- it's legally registered to the head of security for his tour ... who was in a hospital bed the night police seized the weapon, TMZ has learned.

TMZ broke the story ... the assault rifle turned up when cops searched Jeezy's bus on Aug. 24 in Orange County in connection with a fatal concert shooting 2 nights earlier. Police say they arrested all 6 people on the bus because no one claimed ownership.

Sources tell us the gun's owner -- the tour's security chief -- had a good reason for not being there ... the guy got shot in the arm at 1Oak nightclub the night before. We're told he was caught in the crossfire when Suge Knight got shot 6 times ... and TMZ got video of him leaving the club with his wounded arm all wrapped up.

We're told there are serious questions about Jeezy's arrest. According to law enforcement sources the AK-47 wasn't found anywhere near him. It was in a closed case in an area associated with someone other than Jeezy. Meanwhile, he was in an entirely separate cabin of the bus where NO weapons were found.

Official reports related to the arrest indicate the arresting officers were repeatedly told the registered owner of the weapon was the tour security chief, and that he was hospitalized.

Jeezy's been busy on tour since posting bail in the puzzling arrest. Thursday night he was on the 'Tonight Show' ... and even shouted out Joan Rivers.