Just what were the North Koreans firing at Yeonpyeong Island, and from what artillery? It's likely that some or all of the fire was from their variant of Russia's BM-21 Grad MLR, short for "multiple rocket launcher" system. These are angle-adjustable banks of launching tubes on a rotating turntable; all this bolted to the back of a flatbed truck. The Grad and its knockoffs are highly mobile and quick to set up for firing; while limited in accuracy, they can launch a fearsome barrage of 40 warheads in just a few minutes, striking targets more than 20 miles away, depending on the model. Then the crew either slaps on a reload pack with 40 more rockets, or if counter-battery fire is on the way, hits the road and hunts shelter.
Allegedly the North Koreans rehearsed the attack to time their fire (which originated from multiple locations) in such a way that the projectiles struck the island at about the same time. In military terms time-on-target coordination is an effective anti-personnel tactic since troops (and civilians, here) don't have time to seek shelter.
Each Grad rocket is about ten feet long including the propellant section, and 122 mm in diameter, or a little more than five inches across. That makes for a big warhead, holding about fifty pounds of payload.
According to an article in the Daily Yomiuri, dud rounds indicate that the warheads held a powerfully engineered explosive called a thermobaric weapon; aka fuel air explosive. ("Thermo" stands for heat, "baric" for pressure.) While engineering a fuel-air explosion from a fast-moving warhead is not easy, thermobaric rounds can be very effective in military terms, and now come in sizes large and small. A unit of Marines reported in 2003 that a relatively small 40mm thermobaric grenade blew apart a single-story building. Users of the rocket propelled grenade launcher called the RPG-7 can choose from a thermobaric round called the TBG-7V.
The Germans may have experimented with thermobaric explosives in World War II and the U.S. used them against them in Vietnam, such as to clear helicopter landing zones. While still not widely used in wartime (so far) there's nothing secret about the physics of thermobaric explosions, which lie behind every house blown apart in a natural gas explosion, and every factory or grain mill wiped out by a dust explosion. Following somebody's unwise decision in February 2010 to purge pipes at the brand-new Kleen Energy Energy Systems combined-cycle power plant with natural gas, a fuel-air explosion demolished that 620-MWe plant before it even started up.
Some experts regard thermobaric explosives as several times more powerful (pound for pound) than conventional high explosives like Comp B; perhaps up to six times as powerful. One reason: because thermobarics draw their oxygen from the air; they don't have to carry an oxidizer in the warhead, so all the filler can be given over to energetics like volatile liquid fuels and aluminum particles. So thermobarics aren't going to work underwater, in space, or at very high altitudes. To see how conventional explosives are burdened with oxygen, take a look at the RDX molecule diagrammed here.
Characteristic of the thermobaric warhead in action, when viewed in slow motion, is the double explosion. The first charge disperses a superheated cloud of fuel so that it can mix with air; the second and much larger explosion that follows is the fuel-air blast. Here's a film of a big thermobaric bomb being tested.
If Country A wants to attack Country B's vast tunnel complex, count on liberal use of thermobaric warheads after armor-piercing rounds open up the portal doors. The effect of a fuel-air explosion, which is plenty devastating in the open, is greatly magnified in confined spaces if the proper fuel-air mix can be achieved.