“The Third Bullet” by Stephen Hunter; Simon & Schuster ($26.99).Someone should have asked a sniper.That’s what Stephen Hunter — a bestselling novelist who created Bob Lee Swagger, the best shooter there ever was, the Gun Whisperer — believes.The gaping hole in the middle of most Kennedy assassination theories, Hunter says, is that the theorists, even those attached to the original Warren Commission, don’t know the first thing about shooting, ballistics or angles.“Looking at the body of assassination material,” the author says, “too many people who knew too little have said too much.”Hunter’s action-packed new thriller, “The Third Bullet,” in stores Tuesday, puts Swagger on the case and introduces a shockingly plausible alternative to the Lee Harvey Oswald-”lone gunman” explanation.Without giving too much away, it involves a conspiracy of just three people, a sharpshooter hidden in the neighboring Dal-Tex Building and an “exploding” bullet that leaves no forensic trace.“I’m very curious what the assassination community will make of it,” Hunter says.But ultimately it makes no difference if experts try to pick apart his scenario, because there’s a safety net for Hunter: His book is a work of fiction, and is not presented as revelatory findings and gospel truth.“I know what criticism will be leveled at me,” he says. “It’s that I want it both ways, that I want credit for an ingenious theory, but I don’t want to be held too strictly to a standard of truth because this is a thriller. The only answer I have to that: guilty, guilty, guilty.”“The Third Bullet” is a thriller, so it’s possible not only for Swagger to raise interesting questions, but also to solve the mystery and even mete out justice 50 years after the crime.Let’s see your typical JFK conspiracy theorist pull off that trick.