When a film opens with a shotgun blast that interrupts a lousy high-school-band practice on a miserable winter day, the prospects for a happy ending don’t look good. David Gordon Green’s adaptation of Stewart O’Nan’s novel about small-town misery, madness, and folly varies its bleak tone with occasional moments of rueful irony. I laughed, but I was still hoping they’d start the shooting and get it over with. So who’s pulling the trigger? The self-centered science teacher (Griffin Dunne) and the boozy wife (Jeanetta Arnette) he’s two-timing? Or, on the other side of the tracks, the angelic hash-house waitress and single mom (Kate Beckinsale) and her alcoholic Jesus-freak ex-husband (Sam Rockwell)? The competition is stiff, with Rockwell’s character the most contemptible. But they should clear the stage to make room for the more engaging innocent bystanders, like Nicky Katt’s lunkish lothario, Amy Sedaris as his crusty wife, and Olivia Thirlby’s arty girl with a camera, the Wes Bentley of this American Ugly. 106 minutes | Kendall Square