While there are still HEAT rounds (High Energy Anti Tank), the 'state of the art' is a 8 pound depleted uranium (DU) dart.
Early anti tank rounds were solid shot (steel), which would penetrate the relatively thin armor of the early tanks. As the armor got thicker (and hardened), the velocity of the AP shot was increased to defeat the armor. Eventually a point was reached where the AP shot was moving fast enough to shatter against the armor instead of penetrating.
The solution to this was to put a hardened "cap" on the AP shell. Thus, AP became APC (armor piercing capped). As the gun/armor race continued, we eventually reached the point of APCBC rounds (armor piercing capped ballistic capped). And even these had a maximum amount of armor steel that could be penetrated at a given range. The very thick (6"+) frontal armor of late WWII German tanks was proof against many of these rounds, even at point blank range for some Allied tank guns.
At the same time as advances in AP shot were being developed, the HEAT round was also developed. High Explosive Anti Tank (HEAT) rounds used the shaped charge principle. Rather than using kenetic energy to penetrate the tank armor, the HEAT (at one time also known as the "squash head) round strikes the armor, deforms (squashes) into the shape needed for the shaped charge effect, then the explosive detonates, and the shaped charge effect focuses the explosive force into a small area, which "burns" through the armor. HEAT rounds are low velocity (high velocity is both not needed, and actually reduces the effectiveness of the shaped charge warhead), and because of the low velocity can be effectivly fired from light weight weapons like the Bazooka, the German Panzerfaust, and the Soviet RPG, as well as being fired through conventional tank guns. HEAT rounds can be made to penetrate any thickness of armor that a tank can carry.
The Germans quickly recognised a defense against HEAT rounds, thin metal sheets (5mm) spaced some distance away from the main armor of the tank. Since the HEAT round needs a fixed distance to focus the explosive, these "schurtzen
" (skirts) would detonate the HEAT rounds too far out from the hull for them to penetrate the armor. The final versions of the schurtzen weren't even plates, but a heavy gauge metal mesh, which worked just as well. Some Soviet tanks had metal bedspring frames welded to their outsides to get the same protection from the German HEAT rounds.
Today, tanks obtain the same protection from HEAT rounds by using "reactive armor", blocks of explosive attached to the outside of the armor, which explode when hit, disrupting the shaped charge, and preventing it from burning through the armor.
AS mentioned, state of the art today is the DU sabot round. Depleated Uranium is used because it is incredibly dense. Denser than tungsten, ro anything else readily usable. The round is a subcaliber "dart" fired in a sabot from a large bore gun (105/120mm), at very high velocity (5,000fps+). The APFSDS round (Armor Piercing Fin Stabilized Discarding Sabot) relies on kenetic energy (very high) focused in a small area to punch through armor, and it has the added benefit that the Uranium is pyrophoric, and is ignited as it penetrates the armor, destroying the enemy tank, normally with a single shot. They have the nickname "silver bullets" because they kill the werwolves DRT!
As to whether it is heat or "cold plasticity" that causes steel to flow under impact, or barrel obstructions, I can't say, not being as highly educated as some folks. All I can say is that the steel certainly looks
like it was melted!
I shoot a lot of steel plates and I never noticed any melted looking globules. All I see is a flat disc on the ground and a lot of lead smears on the steel.
Hawg, if you shoot steel plates with high velocity rifles, you will see the steel plates crater, looking very much like they were melted.
I don't know what they teach nowdays, but back when I was in school they taught that pressure=heat and heat=pressure in general terms. Somebody mentioned torsion and how the steel reduced diameter before it broke. What they didn't mention was how a torsion breakage also generates a lot of heat.
Call it what you want, under high pressure, steel (and lots of other materials) act like liquid. And they get hot too!