The development of light, man-portable, anti-tank weapons increased during the Second World War. Most were based on the Munroe effect which led to the development of the high-explosive shaped charge. These weapons were called high-explosive anti-tank (HEAT). The destructive effect was reliant entirely on the kinetic energy of the explosion rather than the ballistic speed of the round on the damage inflicted to the armor. The effect was also concentrated and could penetrate more armor for a given amount of explosives. The first HEAT rounds were rifle grenades, but better delivery systems were soon introduced: the British PIAT was propelled in a manner similar to the spigot mortar with a blackpowder charge contained in the tailfin assembly, the US Bazooka and the German "Panzerschreck" used rockets, and the German "Panzerfaust" was a small recoilless gun. The HEAT warhead was retroactively used to give more power to smaller calibre weapons such as in the conversion of the otherwise limited German 37 mm PaK guns to fire a large shell, called Stielgranate 41, that fitted over the barrel rather than down in it, to a greater range than the "Panzerschreck" could manage.