When a legendary man of action is reduced to catching cobras for a tourist show in Thailand, you know something must be simmering beneath the surface.

This is the state of John Rambo at the beginning of “Rambo,” the fourth movie about the much-abused warrior. Could this be Sylvester Stallone’s autobiographical statement about his movie career? After all, it’s been a while since Stallone scored big — at least until 2007’s surprise comeback outing, “Rocky Balboa.”

Stallone, who also co-wrote and directed “Rambo,” held nothing back for this sequel. It’s a wall-to-wall gorefest of severed limbs, exploding guts and crushed skulls; even by the standards of modern-day action flicks, it’s astonishingly violent.

We don’t learn much about what John Rambo has been up to since his last movie appearance (1988’s “Rambo III,” his adventure in Afghanistan), but he now lives in Thailand, running a boat along a river near the border with Burma (also called Myanmar).

As though to prepare us for the carnage to come, Stallone begins the movie with real footage of people wounded by the effects of war in Burma. It’s very odd to think that many viewers might become aware of the dire situation in Burma because of a Sly Stallone action picture.

As usual, Rambo doesn’t talk a whole lot. When a Christian group from the States approaches him to ferry them into Burma on a humanitarian mission, he’s terse and skeptical.

But he does it, and the bullets begin flying shortly thereafter. Arrows, too; Rambo’s use of a bow and arrow at a critical moment is one of the movie’s first big jaw-droppers.

They come fast and furious after that. Give Stallone credit: The rescue plot may be crude, but it gets the blood racing. The villains are despicable, and the mercenaries coming to get them aren’t much better.

The action is anything but conventional. There hasn’t been this much attention paid to individual scenes of bodily harm since the original “Dawn of the Dead.”

The violence is so extreme, it’s almost as though Stallone is sneering at these young pups making “torture porn” movies, and showing them how a man over 60 does it. And he doesn’t even have to take his shirt off.

For that, I thank him. And good to know that Rambo still interrupts his moody silences with the occasional epigram — who could forget “Do we get to win this time?” from the Vietnam trek in “Rambo: First Blood Part II.” This movie’s “Live for nothing, or die for something” might not have the same ring, but at least he’s still out there trying.