Originally posted by StratComm
Nope, not the case at all. AP rounds are by their very nature relatively soft, believe it or not. A diamond would shatter on impact. Copper is used because copper turns to a molten form at a low enough level to be reached by conventional explosives, and is malliable enough to stay in one cohisive blob as it penetrates the armor.
The idea of an armor-piercing weapon is not to put the whole shell through the armor plating, just to put something so deadly to the crew through that no one is still in the tank to run it when you're done. AP weapons detonate on the surface of armor, propelling a small slug (often made of copper or some other relatively soft metal) into the armor. As the slug penetrates, it generates a lot of heat, so that by the time it reaches the cabin beneath the armor it is superhot and quite lethal. It then (in theory) spews onto the crew or the ammo or something like that and takes the tank out of action. The crew (or some other flammable, non-explosive type thing) getting hit results in a burnt-out tank, and if the ammo stores get hit then you're liable to see a popping turret. Nasty things armor piercing weapons are, but not exactly made up of what you might think.
Technically speaking, nobody uses straight 'AP' any longer - it's either APDS sabot rounds or HEAT rounds.
The former use extremely dense, hard metals such as tungsten or uranium to mechanically push their way through armor by maximum force on minimum area - that's why APDS penetrators are only about 40-50mm in diameter, about a third of the gun bore, and are extremely long relative to their diameter. These are the most reliable at penetrating armor because there aren't so many nifty tricks you can do to stop them except add more armor and slope it.
HEAT arounds use a shaped charge to fire a jet of superheated metal (as stated, normally copper) through armor, cutting through armor with a combination of thermal and kinetic energy. These weapons are handy because you don't need a big ****ing gun to fire a HEAT round, the speed of the shell has no relevance to the effect of the warhead, hence why they're used on missiles and rockets. However, there's more ways to defeat them, such as ERA, composites and some experimental stuff like the electric armor.
