It is one of the enduring mysteries of American movies: why has no one yet figured out a way to let Sylvester Stallone be as funny onscreen as he is in interviews? (And don’t even think of mentioning “Stop! Or My Mom Will Shoot.”)

No one is more self-deprecating and charmingly funny selling his latest megaplex product than Sylvester Stallone. He was absolutely delightful on David Letterman’s show pushing the winsomely titled “Bullet to the Head.”

And yet in a community and town loaded with talent, no one in Hollywood knows how to harness that self-mocking charm in a comedy. (Stallone himself, playing the stumblebum and leg-breaker in the stingy-brim hat in the first and best hour of the original “Rocky,” probably came closest. Then the boxing heroics ruined it – forever, it seems.)

The audience for “Bullet to the Head” wants so much to laugh WITH its beloved Sly that it winds up laughing AT him just as often. The movie only gives him about seven or eight negotiable wisecracks, so the audience fills in its own merriment over what seems to be abundant, unintentional self-parody by America’s 66-year-old steroid superstar.

It’s too bad, really. It isn’t Stallone’s age that works against him in “Bullet to the Head,” it’s our sophisticated understanding of Stallone’s own knowledge of exactly how silly it is to be swaggering, muscle-bound and ramrod straight through a movie like this. We want to party with Sly; when the movie refuses to let us, we party at his expense.

Put another laconic musclehead – Jason Statham would be perfect – in the lead role and it would be a convincing cinematic action comic. With Stallone in the lead, it’s a big missed opportunity.

Action comic book is essentially what the movie is – a movie version of a “graphic novel” (i.e. comic book) by Alexis Nolent called “Du Plomb dans la tete” (literal translation: lead shot to the head).

Stallone plays James Bonomo, aka “Jimmy Bobo,” a hired killer from New Orleans whose young assassin buddy is murdered by a hulking, pony-tailed monster who seems to love nothing more than slaughtering his fellow humans. Pony-tail monster is played by Jason Momoa, a young actor who can match Stallone muscle for muscle, which is why they tried to get away with starring him as the new Ah-nuld in a 2011 “Conan the Barbarian” remake. He’s much better known for being the TV barbarian in HBO’s “Game of Thrones” who had to be elaborately tutored by his wife in the civilized practice of face-to-face sex.

Stallone’s hit man thinks nothing of killing-for-hire in a hotel room, but will occasionally draw the line at also gratuitously killing a hooker who might be in the shower at the time. “The people I work for are s---,” he tells us. “The people I take out are worse.”

All that comic book vigilantism is just in case you’re having trouble warming up to the idea of lovable hit men. When his partner (Jon Seda) is dispatched by Pony Tail, a Washington D.C. cop (Sung Kang) offers to team up with Bobo to get the bad guys they both want.

The cop, of course, wants to claim them for the courts and the law. The killer just wants the bad guys dead and all previous debts squared away.

Bullets fly. Bodies pile up – eight in one fell swoop when Pony Tail single-handedly marauds a competitor’s restaurant. One could, I suppose, high-mindedly deplore all this as contributing to our current gun insanities, except that the comic book unreality of it all couldn’t be processed by anyone “normal” as anything remotely life-like.

What’s most interesting about the movie is that it’s directed by Walter Hill, a great veteran action-movie maestro whose 1970s movies “The Warriors” and “The Driver” are absolutely congruent with all the graphic novels and kung fu extravaganzas that followed.

This is very much a Walter Hill movie, even though Stallone, no doubt, is the reason it was made.

Unfortunately, Sly would have done better if he’d found a way to be less active and funnier elsewhere.