From another web site.
The Pacific War Online Encyclopedia: Bombs
General-purpose or demolition bombs had thin metal walls, a large explosive charge, and a contact fuse that was triggered by a relatively light impact. Such weapons were effective against soft targets, which included unarmored
ships,
infantry in the open, and most civilian installations.
The Japanese
D3A "Val" usually carried the Type 99 Number 25 Model 1 Ordinary general-purpose bomb, which was designed for attacks on unarmored or lightly armored ships. This bomb weighed 250kg (551 lbs) of which about 62 kg (136 lbs) was high explosive. Its fuse was initiated by impact on ordinary steel plating and had a 0.2 second delay. This allowed the bomb to penetrate 20 to 40 feet (6 to 12 meters) before detonating. However, it had a somewhat high dud rate, with perhaps 27% of those used at
Pearl Harbor either failing to detonate or giving low-order detonations.
Australian troops at
Rabaul in January 1942 also reported a high dud rate. Its American counterparts were fused to detonate after just 0.1 seconds in order to tear up
carrier flight decks. The Americans estimated that three hits with 500 lb (227 kg) GP bombs would almost certainly sink a 1630-ton
destroyer and had a 70% chance of sinking a 2100-ton destroyer. Three hits with 1000 lb (454 kg) GP bombs had a 95% chance of sinking a relatively lightly protected
Atlanta-class
light cruiser and a 30% chance of sinking a 10,000-ton
heavy cruiser. Six hits with the 1000-lb bomb gave a 80% chance of sinking a heavy cruiser"
"
Armor-piercing bombs had thick steel cases, a relatively small explosive charge, and a delayed fuse that did not detonate the explosive until a few tenths of a second or more after the bomb hit a substantial solid surface. This gave time for the bomb to penetrate before detonating. AP bombs were useful against armored ships,
bunkers, concrete
runways, and other hardened targets.
The Japanese apparently did not develop an armor-piercing bomb until 1941. The Type 99 Number 80 Mark 5, used at Pearl Harbor, was remanufactured from obsolete 16" battleship shells. Out of a total weight of 800 kg (1760 lbs), the bomb had a charge of just 23 kg (50 pounds) of Type 91 explosive. The Type 99 had two base fuzes with 0.2 second delay that were insensitive enough to require impact on armor plate for fuze initiation. One such bomb went clear through
Vestal at Pearl Harbor to explode on the harbor floor under the ship. The Type 99 was judged capable of penetrating 5.75" (146mm) of deck armor when dropped from a height of 10,000 feet (3000m). Unfortunately for the Japanese, the Type 99 had a high dud rate, due to weaknesses in the bomb case introduced during the remanufacturing process and poor quality control with the explosive fill. Of those that scored hits at Pearl Harbor, 20% failed to explode and another 40% yielded only low order explosions. The Japanese never developed an armor-piercing bomb light enough to be carried by a
dive bomber, probably because they did not believe it was possible to develop a lighter AP bomb capable of penetrating battleship deck armor, and possibly also because of the shortage of the high-quality steel required for such weapons. "