The recent introduction of the 500 pound smart bomb, and the looming introduction of the 250 pound smart bomb (or SDB; Small Diameter Bomb), are part of a trend towards pinpoint destruction, and less collateral damage (which can be to friendly troops as well as civilians.) For several decades, the most popular bomb was the 2,000 pound version. Even when laser guided bombs were introduced in the late 1960s, the 2,000 pound bomb was preferred because its enormous destructive effect insured that a near miss still destroyed the target.
The damage done by a 2,000 pound bomb is considerable. Most of the bomb is metal, but inside is 945 pounds of high explosive (Tritonal, a version of the old standby, TNT). The immediate effect of the Trinotal going off is the creation of a fireball, heated to about 8,500 degrees Farenheit. High speed photos have shown that the 355mm (14 inch) diameter bomb casing doubles in size before flying apart, sending metal fragments off at the twice the speed of rifle bullets. But because of these fragments are odd shapes, the majority loose their fatal speed, and fall to ground, within about 1,100 meters of the explosion. Some larger fragments have gone as far as 2,500 meters.
The explosion will, on average, dig a crater six meters wide, and throw up five tons of rocks and dirt. Some of the rocks set in motion will be going fast enough to cause injury. But more likely to cause injury is glass or structures within a few hundred meters of the detonation. Glass fragments, and pieces of buildings or vehicles, will cause injuries out to 500 meters and beyond.
The primary guidance system for a JDAM smart bomb is GPS which will have the bomb going off within 13 meters of the aiming point. If GPS is jammed, or fails, theres a back up, unjammable, inertial guidance system that will bring the bomb down to within 30 meters of the aim point. But since the 2,000 pound bomb pretty much obliterates everything within a hundred meters of the explosion, thats close enough. However, with either guidance system, a 2,000 bomb is usually overkill, especially in a built up areas. Thus the preference for 500 pound JDAMs for urban targets. The 250 pound SDB, with its ground penetrating capability, will be even more useful inside of cities. The 2,000 pound bomb will stay in inventory, as it is still the weapon of choice against enemy trenches and bunkers out in the open, or very large structures. But most targets can be destroyed with a smaller bang.