system. Grenade launchers using this technology include the XM29, XM307, PAPOP, Mk 47 Striker, XM25, Barrett XM109, K11, QTS-11, Norinco LG5 / QLU-11 and Multi Caliber Individual Weapon System. Orbital ATK developed air burst rounds for autocannons. Nuclear weapons The air burst is usually above the hypocenter to allow the shockwave of the fission or fusion driven explosion to bounce off the ground and back into itself, combining two wave fronts and creating a shockwave that is more forceful than the one resulting from a detonation at ground level. This "mach stem" only occurs near ground level, exists around the entire perimeter of the expanding wave front near ground level, and is similar in shape to the letter Y when viewed from the side (see sliced view). Airbursting also minimizes fallout by keeping the fireball from touching the ground, limiting the amount of debris that is vaporized and drawn up in the radioactive debris cloud. For the Hiroshima bomb, an air burst above the ground was chosen "to achieve maximum blast effects, and to minimize residual radiation on the ground as it was hoped U.S. troops would soon occupy the city". Some nuclear weapons have a contact preclusion fuzing feature to prevent the backup contact fuze from detonating the weapon if the air burst fuzing fails. Tactics In conventional warfare, air bursts are used primarily against infantry in the open or unarmored targets, as the resulting fragments cover a large area but will not penetrate armor or field fortifications. In nuclear warfare, air bursts are used against soft targets (i.e. lacking the hardened construction required to survive overpressure from a nuclear explosion) such as cities in countervalue targeting, or airfields, radar systems and mobile ICBMs in counterforce targeting. Killer Junior and Senior Killer Junior and Killer Senior are techniques of employing artillery direct fire air bursts, first developed during the Vietnam War. The technique involves a howitzer firing a high explosive (HE) shell using a mechanical time–super quick (MTSQ) artillery fuze set to cause an airburst over a target in very close proximity to the firing gun's position. Set properly, the shell would detonate approximately above the ground at ranges of 200 to 1,000 meters. The term Killer Junior was applied to this technique when used with 105 mm or 155 mm howitzers, and the term Killer Senior applied to its use with the M115 203 mm (8-inch) howitzer. The