In less than a year of service, the new South Korean K21 infantry vehicle has suffered two sinkings as K21s used their amphibious capabilities. One soldier was drowned. Such sinkings are not unusual, as attempts to make these armor vehicles float, while theoretically possible, never work out all that well in practice. The South Koreans believe that the problem is just some minor flaw in the vehicle design, but history of amphibious armored vehicles indicates the major problem is designer hubris. South Korea originally planned to buy about a thousand K21s, but that has since been cut to 450.
The K21 IFV (Infantry Fighting Vehicle) is a 26 ton vehicle, similar to the 33 ton U.S. M-2. The K-21 has a crew of three and carries nine infantry in the back. The two man turret carries a 40mm autocannon, with 200 rounds, as well as two ATGM (Anti-Tank Guided Missile) launchers and a coaxial (with the 40mm) 7.62mm machine-gun. Top speed is 70 kilometers an hour.
The big difference between the K21 and M-2 is weight, and that is mainly because the chassis of the K-21 is made of fiberglass. This material is as strong as metal, but much lighter. This led the South Korean designers to believe that they had a safe amphibious armored vehicle. Armor protection of the two vehicles is equal. The K21 armor can stop 14.5mm bullets and most artillery fragments (it is vulnerable to 152mm shells exploding closer than 10 meters, but at that distance, the blast effect would do serious damage).
The 40mm autocannon fires up to 300 rounds per minute, at speeds of 1,000 meters per second (3,100 fps). The anti-tank round can penetrate up to 220mm of armor. The 40mm can also fire a multipurpose shell, where the gunner can select proximity (exploding close to a target), air burst (exploding above the ground), as well as armor-piercing or fragmentation effect.
Like the M-2, the K21 carries an extensive load of electronics, for fire control, navigation and diagnostics. The K21 designers learned from other recent IFV designs and attempted to improve on their best characteristics. The K21 costs less than $3 million each. But the K21 is not expected to have much more protection from roadside bombs than the M-2. This may prove to be an intractable problem, given that the MRAP gets its protection from its V shaped underside, specific design elements that cannot be incorporated in a vehicle like the K21.