Ex-L.A. Rams WR Deon Long -- who was featured on "Hard Knocks" when he got cut for bringing a woman to his dorm room -- is now a suspect in an armed robbery, TMZ Sports has learned. 

Long's firing became a viral moment when then-coach Jeff Fisher ripped him for violating the team's "no female guests in the room" rule during training camp. 

Now, law enforcement sources tell us the 26-year-old is suspected of pulling a gun on a man in a craigslist transaction that had gone bad over a duffel bag. 

We're told the alleged victim told police he met up with Long to sell him the bag -- but instead of producing cash, Long whipped out a pistol and made off without paying. 

Cops are now investigating. We've also reached out to Long for comment, so far no word back.