You called?
Yes, WW2 cannon shells (and most still today) were hollow, with the contents filled with high explosive or incendiary material (and usually both together). The contents were set off by a fuze, which in the case of aircraft guns was usually screwed into the nose. Early in the war these detonated on impact, later a slight delay action was incorporated so that the shell blew up inside the aircraft.
Various kinds of fuzes were used. The most common were complex mechanical devices. In some cases, they worked by having something like a firing pin which was driven back onto a small impact-sensitive charge and set it off - this in turn set off the shell contents. In other cases they worked by inertia: the sudden deceleration as the shell hit something caused an element in the fuze to move forwards against a spring pressure, also hitting the igniting charge. The fuzes were prevented from functioning when the gun was fired by using a safety lock which could only be unlocked by being spun rapidly - which is what happened when the shell was fired up the barrel and rotated by the rifling. The fuze was usually designed to take long enough to "arm" that the shell was well clear of the barrel before that happened, but sometimes it didn't work properly, leading to "prematures"; in the worst case these could burst in the barrel. Some fuzes (especially those used by the Luftwaffe in home defence) also had a timer which provided a self-destruct function; they blew up once they were out of normal range, usually at 1,000+ metres, to ensure that they caused no damage when they fell to earth.
Another type of fuze was the "air column" type (used in RAF Hispano ammo, and in some Japanese shells). This had no moving parts but consisted of a narrow tube running from the tip to the igniting charge. On impact at high velocity, the air in the tube was violently compressed which caused instant heating and set off the charge.
In smaller calibres, the Japanese didn't bother with a fuze at all: they just filled a thin metal nose cap with highly sensitive PETN HE. This wouldn't explode when the gun was fired, but it did detonate on impact, so it was a good idea not to drop one on its nose...
The most unusual type of fuze saw limited use with the Luftwaffe towards the end of the war. It was a hydrostatic type, designed to be triggered by liquid pressure - such as in a fuel tank.
Most shells in WW2 didn't hold that much HE/incendiary (about 10% by weight of the shell) because the shell body was made by boring it out, and the walls were quite thick. Even so, that was enough to do a significant amount of damage, partly by blast and fire but also by the fragments of the shell body being blasted at high velocity through the plane.
In 1940 the Luftwaffe introduced the Minengeschoss or mine shell, which was made by drawing the metal from a flat disk, just as cartridge cases are made. This resulted in a shell with very thin walls so it was lighter and held more HE (about 20% by weight). There was more blast but less fragmentation, which the Luftwaffe considered to be more effective.
Generally speaking, HE shells were most useful in calibres of 20+mm, although the Germans, Italians and Japanese used them in 12.7-13mm calibre (around .50"), and the Japanese even had 7.7mm (.303") HE bullets. The USA used incendiary bullets in .30" and .50", but never HE.
Hope this helps.
Tony Williams
Homepage:
http://www.quarry.nildram.co.uk
