Armour-piercing fin-stabilized discarding sabot

Despite practical penetrator and target materials not being fluids before impact, at sufficiently high impact velocity even crystalline materials begin to behave in a highly plastic fluid-like manner, so many aspects of hydro-dynamic penetration do apply.[4][5]

Long rod projectiles penetrate a fluid in the literal sense, based simply on the density of the target armour and the density and length of the penetrator. The penetrator will continue to displace the target to a depth of the penetrator length times the square root of the penetrator to target densities. One observes immediately that longer, denser penetrators will penetrate to deeper depths, and this forms the basis for the development of long-rod anti-armour projectiles.[4]

The important parameters for an effective long-rod penetrator, therefore, are very high density with respect to the target, high hardness to penetrate hard target surfaces, very high toughness (ductility) so the rod does not shatter on impact, and very high strength to survive gun launch accelerations, as well as the variabilities of target impact, such as hitting at an oblique angle and surviving counter-measures such as explosive-reactive armor.[4]

While penetrator geometry has adapted to reactive armour counter-measures, tungsten heavy alloy (WHA) and depleted uranium (DU) alloy continue to be preferred materials. Both are dense, hard, tough, ductile, and strong; all exceptional qualities suitable to deep armor penetration. Each material exhibits unique penetration qualities that may make it the best choice for a particular anti-armor application.

Depleted uranium alloy, for example, is pyrophoric; the heated fragments of the penetrator ignite after impact in contact with air, setting fire to fuel and / or ammunition in the target vehicle, contributing significantly to behind-armour lethality. Additionally, DU penetrators exhibit significant adiabatic shear band formation. A common misconception is that, during impact, fractures along these bands cause the tip of the penetrator to continuously shed material, maintaining the tip's conical shape, whereas other materials such as unjacketed tungsten tend to deform into a less effective rounded profile, an effect called "mushrooming". The formation of adiabatic shear bands actually means that the sides of the "mushroom" tend to break away earlier, leading to a smaller head on impact, though it will still be significantly "mushroomed".

Tests have shown that the hole bored by a DU projectile is of a narrower diameter than for a similar tungsten projectile. Although both materials have nearly the same density, hardness, toughness, and strength, due to these differences in their deformation, depleted uranium tends to out-penetrate an equivalent length of tungsten alloy against steel targets.[6] The use of depleted uranium, in spite of some superior performance characteristics, provokes political and humanitarian controversy, but remains the preferred material for some countries due to lower cost and greater availability than tungsten. Tungsten itself has been found to be biologically hazardous and creates exposure hazards only somewhat milder than depleted uranium.

Typical velocities of the APFSDS rounds vary between manufacturers and muzzle length/types. As a typical example, the American General Dynamics KEW-A1 has a muzzle velocity of 1,740 m/s (5,700 ft/s).[7] This compares to ~914 m/s (3,000 ft/s) for a 5.56mm round fired from an M16 rifle. APFSDS rounds generally operate in the range of 1,400 to 1,800 m/s (4,593 to 5,906 ft/s). Above a certain minimum impact velocity necessary to overcome target material strength parameters significantly, penetrator length is more important than impact velocity; as exemplified by the fact that the base model M829 flies nearly 200 m/s (656 ft/s) faster than the newer model M829A3, but is only about one half the length, wholly inadequate for defeating state-of-the-art armor arrays.

Complicating matters, when foreign deployment of military forces or export sales markets are considered, a sabot designed specifically to launch a DU penetrator cannot simply be used to launch a substitute WHA penetrator, even of exactly the same manufactured geometry. The two materials behave differently under high pressure, high launch acceleration forces, such that entirely different sabot material geometries, (thicker or thinner in some places, if even possible), are required to maintain in-bore structural integrity.

Often the greater engineering challenge is designing an efficient sabot to successfully launch extremely long penetrators, now approaching 80 cm (31 in) in length. The sabot, necessary to fill the bore of the cannon when firing a long, slender flight projectile, is parasitic weight that subtracts from the potential muzzle velocity of the entire projectile. Maintaining the in-bore structural integrity of such a long flight projectile under accelerations of tens of thousands of g's is not a trivial undertaking, and has brought the design of sabots from employing in the early 1980s readily available low cost, high strength aerospace-grade aluminums, such as 6061 and 6066-T6, to high strength and more expensive 7075-T6 aluminum, maraging steel, and experimental ultra-high strength 7090-T6 aluminum, to the current state-of-the-art and expensive graphite fiber reinforced plastics, in order to further reduce the parasitic sabot mass, that could be nearly half the launch mass of the entire projectile.[7]

The discarding sabot petals travel at such a high muzzle velocity that, on separation, they may continue for many hundreds of feet at speeds that can be lethal to troops and damaging to light vehicles. For this reason, tank gunners have to be aware of danger to nearby troops.

The saboted flechette was the counterpart of APFSDS in rifle ammunition. A rifle for firing flechettes, the Special Purpose Individual Weapon, was under development for the US Army, but the project was abandoned.