A Quick Overview
Ever since someone boiled a hide to help protect against hostile spears, the other someone make the spear pierce a little better. Through the ages of metal and industrialization, someone has always tried to get through someone else’s armor. Wargames have dealt with the concept of armor verses penetration on many scales. On all but the most tedious, the back and forth of that competition is usually left out. Here is an overview of what goes on below the level considered in Panzer Grenadier in regard to penetrating the boiler plate vehicles began to bolt on themselves in World War 1.
Basically there are two ways to stop a tank. Poking a hole in it is the first and most common method. The second is to put a big enough bomb on the vehicle to damage it; hopefully, a lot of damage. Penetrating metal armor plate was initially a naval problem, but moved on land with the introduction of steel louvers to cover the firing ports of fortifications. These were the solutions to penetrating armor, before the armies of the world went to slip-rings, fin-stabilized darts, and depleted uranium.
The basic Armor Piercing (AP) round was invented in the Austria-Hungary in 1905 for use in the Mannlicher 1890/31 Infantry Stützen Rifle. It was simply an 8×56mm steel bullet with a lead sleeve. Later armor-piercing bullets had a cupro-nickel jacket. These bullets met the British and French tanks that crossed the trenches of World War One. A solid shot was the basis of cannon ammunition used against tanks. Large gun rounds were able to incorporate a bursting charge. All of them work on the same basic concept: hitting armor with a hard, sometimes very large bullet.
As antitank guns grew in size, it became apparent that the best shape for penetrating armor was not quite the best ballistic shape. The solution was to put a light windscreen on the AP round. This ammunition was known as Armor Piercing, Ballistically Capped (APBC). The concept was that the shot travels further and faster. The degradation caused by smashing the windscreen was minimal compared to the effect of the retained velocity.
As shot velocity increased, a peculiar problem began to manifest itself. Solid shot, made of steel or steel alloy, would sometimes shatter when striking armor at very high velocity. The British army was especially plagued by this in their 2-pounder and 6-pounder guns when confronting German tanks with face-hardened armor. The solution was another “cap,” this time of lead. The result was referred to as Armor Piercing Capped (APC). What the lead provides is shock absorption, acting in effect like cutting fluid. The same effect happens with glass; drilling glass in the air shatters it, drilling in water prevents that. It seems to have first been applied in France with their 25mm gun, which fired a very high velocity shot.
The problem with APC was that it was shaped like a plug. It didn’t help much that the lead would deform slightly when fired, or pick up dings in the field. The next improvement melded the previous two. The shot was capped with lead (or whatever), then given a ballistic cap. The result was called Armor Piercing Capped, Ballistically Capped (APCBC). This is what is now thought of as solid shot. It is also the end of basic armor-piercing rounds. From here this starts to get more exotic.
The basic physics of armor-piercing shot are proportional to mass and velocity, inversely proportional to the cross sectional area, and modified by dozens of other things. One those things is metal quality. Despite the name, “bad silver,” tungsten is an excellent metal for an antitank round. It is more than twice as dense as steel and is more resistant to shattering. Putting a tungsten slug inside of a light weight jacket resulted in a round of similar initial mass, but delivering greater kinetic energy on impact. This type of round is called Armor Piercing, Composite Rigid (APCR). The operational theory is when the round strikes the target, the tungsten slug breaks through the jacket and, with its lower cross section and near equal mass, punches through thicker armor. The Germans introduced this type of ammunition in late 1941 against Soviet tanks; the Russians fairly quickly learned to make their own.
Quantumly speaking, the world is perpetually imperfect. Try as one might the slug of an APCR round is not truly in the center. Any round fired from a rifled barrel will wobble. An APCR round will wobble more, with its light outer material and its (however so slightly) off-center heavy core. An obvious solution to overcome this is to not use a rifled barrel. But how to stabilize the shot? The solution became very tightly controlled trajectory, with a very high-velocity launch. Introducing the squeeze bore gun and Armor Piercing, Composite Non-Rigid (APCNR) ammunition. The base concept is a hard core with a soft sleeve outer jacket. When fired, the shot travels down an ever-narrowing barrel. The soft outside of the shot deforms and folds behind the core, exiting the barrel at tremendous velocity. These sort of guns were invented by the Germans. Fortunately the Germans did not have the spare tungsten to use on ammunition; consequently by late 1942, most ammunition had to go back to capped steel alloy. The British saw the value in the concept and developed the Little John adapter for the 2-pounder antitank gun. The Little John kept the 2-pounder relevant until the end of the war (mostly in armored cars).
Tapered bores and APCNR were not the only solution for getting high density shot to a target. In 1940 French engineers were working on the problem of getting high density shot down range. They decided that all that was important was to get the sub-caliber shot there; anything else was superfluous. Their idea was to remove the light jacket material from the shot after firing. The spin caused by the cannon’s rifling would provide the force to remove the jacket, or as the French called it, “sabot,” meaning boot. The round is called Armor Piercing, Discarding Sabot (APDS). With the jacket/sabot gone, the spin of the shot was centered entirely on the higher density material, which meant less wobble. That meant the strike was more direct, which in turn meant it penetrated better at longer ranges than APCR. The French engineers, now evacuated to England, refined the design. The first ammunition was issued for 6-pounder guns in mid-1944; 17-pounders began to get their issue in early fall of the same year. The Americans also adopted APDS for use in 76.2/L56 guns, and later for the 3-inch AT guns, but were unable to produce a reliable round until 1944.
Sometimes, you have to poke a lot of holes in the tank.
As mentioned in the beginning, there are two ways to defeat armor. So far, it has been about poking holes. Now we turn to shock. The simplest shock method is putting a High Explosive (HE) charge on or against the target. If the charge is large enough, the tank is destroyed, or the crew is concussed. Delivery of the charge may be by any method: cannon, satchel charge, or airplane. The base operandi is the same, to put a chunk of explosive on the tank and set it off. With enough explosive, even the “on” part isn’t required.
A big kaboom is really not the most efficient way to take out a tank. Back in the 18th century it was discovered that if a scoop was taken out of an explosive charge, an explosion would focus into the hollowed-out portion. A United States naval engineer weaponized the concept in 1888. The shell was later adapted to both guns and rockets. The warhead was referred to as High Explosive Anti-Tank (HEAT). Conceptually, part of the explosion directs the force and gasses from the detonation against the armor for a concentrated effect. The first HEAT shell was made for the 75/L24 on PzIV tanks. It would still be 1942 when HEAT ammunition became generally available. The most famous application of the HEAT warhead was probably the U.S. Bazooka, an antitank rocket launcher introduced during Operation Torch.
One more explosive antitank solution appeared in World War 2. In 1944 the British introduced the High Explosive Squash Head (HESH). A peculiar shell that never went much beyond the Commonwealth armies, it stayed in their ordinance through the Cold War. An early observation in antitank warfare was that partial armor penetration caused armor fragments to fly around inside the vehicle. HESH paints the phenomena on a larger canvas. The shell is a blob of plastic explosive with the detonator in the back. When a HESH round strikes its target, the explosive flattens out, then the detonator makes contact with the target's surface. The resulting explosion does not usually penetrate the target's armor; it fragments and pops off chunks of armor inside the vehicle. Shock, rather than destruction, is the method used here. HESH was first used by the Churchill AVRE as bunker buster. The regular armored forces adopted it soon after.
This pretty much wraps up what happens under those red AT factors you see in the lower corner of the counter. Further refinements and developments would come during the Cold War. Those are beyond the scope of this article. For our frame of reference, these are quite enough.