British actor Ben Barnes was back in Boston Monday for the premiere of the indie flick “By the Gun,” a made-in-Massachusetts mobster movie costarring Harvey Keitel, Leighton Meester, and Stoughton’s Kenny Wormald. (Keitel and Meester couldn’t make it, but the guest list did include Playboy’s Ciara Price, DJ/producer Clinton Sparks, and Ben Mezrich. Monday’s screening at AMC Boston Common was a benefit for the Greg Hill Foundation.