The recent primary function of main battle tanks is to destroy other tanks. In order to destroy a tank, critical components on the inside of the armor must be sufficiently damaged. Accordingly, most tank main gun ammunition is designed to penetrate tank armor. Generally there are two ways to do this; either hit the armor with something so hard and so fast that it punches right through, or hit the armor with an explosive that blows its way through. Armor Piercing (AP) rounds take the first approach and High Explosive Anti-Tank (HEAT) rounds take the second.
Armor Piercing Discarding Sabot
Sabot rounds rely on velocity to penetrate. A sabot round is a sub-caliber projectile. The diameter of the projectile is smaller than the diameter of the cannon's bore. The reduced mass allows the energy of the propellant to give the projectile a higher velocity, and the reduced cross section produces less drag. The mass that the penetrator does have is concentrated in a smaller area, imparting more force at the point of contact with its target. The kinetic energy a projectile has when it hits a target is a function of the square of its speed times half of its mass. Preserving velocity by reducing drag is more efficient, therefore, than preserving mass. Mass is, nevertheless, important. That is why the shape of a saboted penetrator tends to be thin and long. The longer the penetrator is, the more mass it has. The tendency to shatter is, however, a practical limit to the length of a penetrator.
The advantage of concentrating as much mass into the point of impact also leads to the use of dense materials for penetrators. Typically, alloys more dense than steel, such as tungsten carbide, are used. Depleted uranium (DU) is also such an alloy. Other than its density, it has two advantages over more conventional materials. When a DU penetrator moves through armor, it doesn't degrade by blunt deformation, but rather, by adiabatic shearing. In other words, it doesn't mush like the tip of a magic marker, it grinds through like a pencil lead in a pencil sharpener. The other advantage of DU is that it is pyrophoric, meaning that as it is ground down, the ground particles ignite when exposed to air. This effect almost guarantees interior damage if a DU penetrator pierces a vehicle's armor.
At left are several images. The top left image is of an M829 series APFSDS-T cartridge. The long, slender shape of the penetrator is easily seen.
High Explosive Anti Tank
HEAT rounds take advantage of the fact that the energy of a detonated explosive is projected at an angle perpendicular (or 'normal') to its surface. In a HEAT round, the warhead is a modified shaped charge. Shaped charges use the Munroe Effect to focus the energy of the explosive. When an explosive is shaped into having a concave or conical surface, when it detonates, the energy moving away from the facing sides of the cone combines, forming a concentrated jet. In HEAT warheads, a metal plate lines the inside of this cone. During the detonation, the metal plate is formed into a rod that is thrust from the shaped charge at hypersonic speed. It takes some distance for this rod to form, so a HEAT round will require a bit of stand-off; generally the required stand-off distance is about twice the diameter of the base of the charge. If the rod penetrates the armor, it, along with any molten bits of the armor, will spray inside the vehicle causing damage.
The cartridge on the top right at the left side of the page is an M830 HEAT round. The copper colored alloy liner is clearly visible. The conical shape of the explosive charge is as well. At the end of the stand-off probe is a contact fuse that leads to detonator at the base of the explosive charge. The overall shape of the projectile is obviously not very aerodynamic, accounting for the low velocity of the round and the fact that it requires a relatively high trajectory. But because of its reliance on chemical rather than kinetic energy, it need only hit its target to cause damage, it need not hit it fast.
This site contains an interesting set of slides modeling the formation of a copper rod in a detonating HEAT warhead: Shaped Charges
Anti-Tank Guided Missiles
Anti-Tank Guided Missiles (ATGM) use HEAT warheads to defeat their targets.
The picture beneath the two tank cartridges is of a Russian/Soviet 9M119/9M119M Svir/Refleks anti-tank guided missile (NATO designation: AT-11 Sniper). The Svir can be shot through the main gun tube of an appropriately equipped T-72, and the Reflecks shot through the gun tube of a T-80 or a T-90. The Svir uses radio command guidance to impart flight correction data to the missile during its flight; the Reflecks missile stays guided onto its target, however, by detecting whether it is within the path of a laser beam that the gunner keeps on the target.
Underneath the AT-11 picture, are two thumbnails of TOW missiles. (These images were taken from the manufacturer's product data sheets available at this website: Raytheon Company: Products and Services: TOW family. Which, by the way, has some nice video. Clicking on the thumbnails will open the product data sheets in new windows.) The TOW missile uses a wire that is spooled out during flight as its command data link. The guidance system compares the location in the sight picture of a beacon on the rear of the missile to the crosshairs and sends corrections to keep it on target.
The TOW-2A has a HEAT warhead, the TOW-2B, however, has a warhead that contains two explosive charges that form downward blasting slugs. This arrangement takes advantage of the fact that most tanks have weak armor on their top. The TOW-2B flies a bit above the point of aim and has sensors that detect when it passes over a large metallic mass such as an armored vehicle.
High Explosive Squash Head
There are types of anti-tank ammunition other than AP and HEAT. High Explosive Squash Head (HESH) or High Explosive Plastic (HEP) rounds are a type of explosive round that can be used to defeat plate armor or blow holes through reinforced concrete.
HESH rounds also take advantage of the fact that the energy of a detonated explosive is projected at an angle normal to its surface. Thus if an explosive is in the shape of a sheet, most of its explosive energy will be directed in only two directions, outward from either face of the sheet. If one side of the sheet is backed with metal plate, then that will direct more of the energy toward the direction of the open face. How a HESH takes advantage of this is that when it hits a surface, it does not immediately detonate, rather it takes a small amount of time to use the force of the impact to deform the warhead, squashing it, so that it broadly contacts the surface. If the surface is armor plate, when the broad surface of the explosive does detonate, it sends a shock wave through the armor that causes internal flaking. The flaking or spall, when projected through the interior of a vehicle, causes the damage. This effect can be defeated by the use of spaced armor to stop the shock wave and by spall liners made out materials such as kevlar placed inside the vehicle.
The British still use HESH rounds with the Challenger II, but that may be phased out as they modify those tanks with the 120mm smoothbore cannon used by the German Leopard II and the American M1A1. The Americans will begin using the HESH round, which they call HEP, as the Stryker Mobile Gun System (MGS) is fielded. The MGS is a Stryker chassis that mounts an autoloaded 105 mm rifled canon. It is intended as a support weapon for infantry and will employ its HEP ammunition to defeat hardened targets such as bunkers and to create openings through walls large enough for infantry to use.