There are basically two ways of piercing armour; kinetic and chemical:
Means chucking a hard thing fast enough at armour to break through it.
Generally, the harder the projectile and the faster you chuck it, the more armour you can pierce. There comes a point where the projectile just shatters when it hits, as the velocity increases. To get the higher velocity, you need to put more propulsive gas behind it. This means you need a bigger breech and longer, stronger barrel.
- HESH, High Explosive Squash Head, is where you have a very thin-walled shell, which you whack onto the target at relatively low velocity. It sits there like a high explosive cow pat. When it goes off, the stresses generated in the armour blast off a scab of the inner wall, which scuttles about the inside of the vehicle creating problems for those who get in the way.
- HEAT, High Explosive Anti Tank, relies on the shaped charge effect to create a jet of very high velocity gas which can penetrate the armour. This effect can be enhanced by lining the cavity of the shaped charge with a thin metal sheet.
- Neither HESH nor HEAT rely on a high velocity delivery to work. In fact, there are armour piercing weapons that are fired from a stationary position, such as the EFP types used by Iraqi insurgents.