A Russian firm is marketing a version of the Klub cruise missile that can be carried in a 40 foot shipping container. The launcher and the missile have to slide out of the container before firing, thus limiting where it can be placed on a ship, particularly your typical container ship. But you could get two or three of these shipping container Klubs on most cargo ships, turning the vessel into warship.
The Klub missile is a key weapon for the Kilo submarine. Weighing two tons, and fired from a 533mm (21 inch) torpedo tube, the 3M54 has a 440 pound warhead. The anti-ship version has a range of 300 kilometers, and speeds up to 3,000 kilometers an hour during its last minute or so of flight. There is also an air launched and ship launched version. A land attack version does away with the high speed final approach feature, and has an 880 pound warhead.
What makes the anti-ship version of the 3M54 particularly dangerous is its final approach, which begins when the missile is about 15 kilometers from its target. Up to that point, the missile travels at an altitude of about a hundred feet. This makes the missile more difficult to detect. The "high speed approach" (via the use of additional rockets) means that it covers that last fifteen kilometers in less than twenty seconds. This makes it difficult for current anti-missile weapons to take it down.
The 3M54 is similar to earlier, Cold War era Russian anti-ship missiles, like the 3M80 ("Sunburn"), which has a larger warhead (660 pounds) and shorter range (120 kilometers.) The 3M80 was still in development at the end of the Cold War, and was finally put into service about a decade ago. Even older (it entered service in the 1980s) is the P700 ("Shipwreck"), with a 550 kilometers range and 1,650 pound warhead.
All these missiles are considered "carrier killers," but it's not known how many of them would have to hit a carrier to knock it out of action, much less sink it. Moreover, Russian missiles have little combat experience, and a reputation for erratic performance. Quality control was never a Soviet strength, but the Russians are getting better, at least in the civilian sector. The military manufacturers appear to have been slower to adapt.
Still, it is unusual for a firm to offer such a weapon for concealed transport on a merchant ship. So far, there have not been any buyers. Or, rather, the manufacturer will not admit to any sales. While these missiles are of questionable effectiveness in wartime, they would likely be much more potent if used for a surprise attack on a military or civilian target.