In “Machine Gun Preacher,” Gerard Butler plays Sam Childers, a gangster who finds God and travels to Sudan, where he builds orphanages and schools while fighting to protect his charges.

With a title as canny as “Machine Gun Preacher,” Marc Forster’s film has surprisingly little to do with the grindhouse genre it evokes. Rather than dabbling in camp or irony, “Machine Gun Preacher” is instead an exercise in full-blown Hollywood mediocrity—a dull drama of big-budget sentimentality. It’s a shame too, considering the potential of the film’s larger-than-life premise.

“Machine Gun Preacher” is based on the borderline-surreal story of Sam Childers (Gerard Butler), a reformed convict and former heroin dealer who found the Christian faith and then moved to Sudan to found an orphanage and battle child-abducting guerillas. However, the mystique of the character of Childers and his story quickly dissolves, as the film instead refocuses on the man playing him. For Butler, coming from previous films like “300,” “Law Abiding Citizen,” and “The Ugly Truth,” being cast in such a humanitarian role could have been an inspired opportunity for more nuanced performance. Yet after watching “Machine Gun Preacher,” it is hard to conceive of the film as anything but another Gerard Butler action vehicle.

And a vehicle it is—as a biopic, “Preacher” covers a lot of ground rapidly, all with Butler on screen for what feels like 90 percent of the time. Too often the movie plays as a listless montage for the transformation of Butler’s character. The film begins with a Mad-Max-dressed Butler almost parodically spouting quotes as awful as “You’re a fuckin’ junkie stripper and you know it!” to his patient wife Lynn (Michelle Monaghan). Soon enough, the gangster finds God and becomes a venerable father wearing crisp button-ups, and not long after he arrives in Sudan. There Childers juggles the responsibilities of his humanitarian work and his merciless hunting of members of the Lord’s Resistance Army.

None of this is to say that Gerard Butler is the reason why the movie is so inauspiciously bland. As the upright yet steely and resourceful alpha male hero, Butler is adept at engaging our sympathies. Rather, at the core of the movie’s malaise are the completely ludicrous scenarios and characters with which Butler has to work. The script, written by Jason Keller, does not do justice to the complicated dimensions and trials of Sam Childers’ mission, nor to Sudan’s bleak history in general. And for an action number, the film has a plenitude of lethargic scenes, many of which seem to exist purely as set-up for individual overwrought one-liners—Hollywood cant like “You’re a mercenary, not a humanitarian!”

Adding to the movie’s misadventures in tone, director Forster occasionally attempts to imbed facile social commentary about modern standards of living and consumption within “Machine Gun Preacher.” The scenes of Sudan are contrasted to back-home Pennsylvania, where Monaghan walks through fluorescent aisles of uniformly arrayed food at a supermarket. At one point, the Childers family attends an opulent fundraising party in a McMansion and the wealthy host snubs Butler’s cause by contributing a paltry $150 instead of the $2,000 requested. Enraged, Butler goes on to deliver a line so wooden for a sentiment so obvious that it is unintentionally hilarious: “$150? He spent more on salsa for his party!” These on-the-nose attempts at modern moralizing fail for the same reason as the rest of the film—its plodding approach squanders the premise’s potential.

Ultimately, convoluted drama and ponderous monotony kill “Machine Gun Preacher.” It is a movie without style or an idea of style. While the film teases viewers with the promise of multiple genres—biopic, documentary, action thriller—it forgoes them all, providing only mechanical and formulaic point A–to-B cinema. Even the title itself is a deception—for a film portending significant deployment of automatic weapons, there is actually very little gun-slinging, and those expecting a very gung-ho and kinetic film will be disappointed. Indeed, “Machine Gun Preacher” is not a movie for people who want an over-the-top military thriller, nor is it for people seeking an ethically and emotionally dynamic true story. It is more for people who want the pretense of one of those things, while Gerard Butler yells things like “I’m gonna get the kids! Cover me!”

Perhaps the most compelling part of “Machine Gun Preacher” comes during its credits, when actual footage of the real Sam Childers is shown. The man is intriguing and older, with large pistols holstered next to his great paunch. He seems a lot humbler than the chiseled, bear-like Butler. One can’t help but wonder if there were a better movie to be made here—if 15 minutes of a documentary on Childers could have been infinitely more fascinating and thrilling than this flaccid half-fiction.