I think the best way to make him good again is to keep him as a relatively bad sentinel.
I would make him a kind of new character, a unique passive. What is Chamber? A rich French gun businessman.
The best way to buff chamber is to give an extra $100 per round for the whole team, after pistol round. Makes it easier to afford OP, gives him a passive use to the team since he's already very selfish. He is strictly an OP-er, nothing more and nothing less. For Ranked, chamber just becomes a lurker with an OP at basically every opportunity. In competitive, teams trying to force a KJ can go back to a Chamber as a semi-substitute but not as a pure replacement. I think this approach works better than improving his ability to OP.