Type 98 320 mm mortar

It later saw action during the Battle of Iwo Jima, Okinawa, and the Soviet invasion of Manchuria. According to Japanese sources its destructive firepower was roughly equivalent to a 203mm heavy conventional artillery piece.

The mortar consists of a steel tube closed at one end by a steel base-plate, which rests on a wooden platform. The 300 kg (660 lb), 1.5 m (4 ft 11 in), 330 mm (13 in) shells fit around and on top of the tube, instead of being dropped inside, making this a spigot mortar. The range of each shot was adjusted by adding different size powder charges at the base of the round. The barrels could handle only five or six shots apiece before becoming damaged and unusable. When used in large groups, as was often done, it produced a fearsome effect known as the "screaming missile" to U.S. Marines. To absorb the massive recoil caused by firing their projectiles, the mortar tubes were almost always placed up against a mound of dirt.

During World War II the Type 98 was first deployed during the Battle of Singapore. The Japanese Imperial Army deployed somewhere between one and two dozen 320 mm mortars on Iwo Jima, as well as two dozen on Bataan. The weapon was also used on Okinawa. One Type 98 was captured by British forces at Imphal. According to Japanese sources the Type 98 was utilized against Soviet forces during the 1945 invasion of Manchuria.

Japanese officers believed the 320 mm spigot mortar's most effective method of employment was to inflict psychological damage on the American troops instead of inflicting casualties. The 300 kg (660 lb) shells left craters 2.5 m (8 ft 2 in) deep and 4.5 m (15 ft) wide, but caused relatively few casualties due to minimal fragmentation. The mortars were operated mainly by the 20th Independent Mortar Battalion. During the Iwo Jima campaign, many of the 12 to 24 launchers were placed inside the mouths of caves to protect them from American artillery bombardment, requiring the gun crews to live in the caves that housed their guns, like the infantry. Due to the relative difficulty involved in moving such a massive weapon system, their locations usually remained fixed during battles.