Heavy-handed message movies don’t come more harrumphing than “Miss Sloane,” a clunky dramatization of the gun-control argument liberals still don’t understand is being conducted solely among themselves.

Jessica Chastain plays a DC lobbyist working to push a bill, aimed at defining and restricting “assault rifles,” that even the filmmakers ultimately concede is meaningless.

Trying hard to be a savvy ballbuster, Chastain’s Miss Sloane comes across instead as shrill and exhausting, a ruthless, stressed-out, all-business career gal whose only intimate relationship is with a male prostitute (Jake Lacy). Naturally, the movie is written by a guy, though not necessarily one who’s acquainted with a lot of women.

“Miss Sloane” spouts many discredited left-wing talking points about firearms, and its NRA-style villains are as deep as an “I’m with . . .” bumper sticker. Even so, give the movie credit for being accurate on one score: In harmony with the current political moment, it sees liberals as attached to the idea that ad hominem attacks, rather than persuading the American people that their policies are wise, are the only way to win, with a skeevy politician (John Lithgow) coming in for targeting.

Clearly inspired by the work of Aaron Sorkin, whose failings are many but who can at least whip up clever dialogue, the film, written by Jonathan Perera and directed by John Madden, veers from preachy monologues to dopey plot twists straight out of daytime television.

Mistaking velocity for intelligence, the cast stampedes through gobs of expository dialogue about lobbying and gun control. They’re so full of bluster, so bereft of convincing detail, that they sound more like the thoughts of a fanboy poli-sci student who just finished a seminar on Sun-Tzu’s “The Art of War” than anyone who’s set foot on Capitol Hill, much less mastered its intricacies.

“Miss Sloane” isn’t a stroll through the corridors of power — it doesn’t even get near the parking lot of power.