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HESH: https://en.wikipedia.org/wiki/High-explosive_squash_head
It was actually developed by the Brits at the very end of WW2, but I don't think actually entered combat by the end of the war. Effectively it is just having a plastic explosive crammed into a thin walled shell with a base detonator that detonate the plastic explosive that 'squashes' against the hull of a tank and transmit the explosive force in a concentrated pat that creates armor rupture and spaling inside the tank, while the shockwave reverberates inside as well, knocking out most of the delicate equipment and human parts inside. Until the advent of anti-spall liners like modern kevlar and spaced armor in the 1970s that are able to contain the shockwave of the blast, it was a highly effective method of knocking out enemy tanks and the Brits still use it for a variety of vehicles, as it is excellent at blowing apart bunkers and buildings, much more so than normal HE rounds.
So what if it was discovered by the two major principle combatants early on, Germany and Britain, and mass produced by 1941? The Soviets couldn't really make it themselves due to how badly disrupted their explosives industry was in the German invasion, though from 1942 on LL could make it available in increasing numbers if the Allies really wanted to give it to them (they didn't give them HEAT shells though beyond Bazookas). So what if all sides with even their stubby short barreled 75s could knock out heavy tanks early on? The Sherman 75mm gun could knock out a Tiger and Panther at shorter ranges (1km or below) if stationary (the rounds had to be relatively slow, below 700m/s in WW2 so the detonator could work right and the shell wouldn't disperse too much if it hits too fast), while even the short 75mm German guns could take out a KV-1 and even a IS-2 (due to armor quality) at normal combat ranges.
How would this impact the war if heavy armor is vulnerable to medium tanks and even relatively short barreled guns? There still would be a market for long ranged heavy guns firing HE, perhaps more so if no armor is safe within 1-2km. Would all sides develop heavy armored AFVs then or would they more focus on medium AFVs that were maneuverable and heavily gunned to try and stay out of range of HESH rounds? Would there be a Tiger/Panther/KV/T-34 panic if there were these rounds that could wreck them easily even with a shorter 75mm gun? How would bunkers and building combat fare given that HE became a lot more effective against infantry in protected positions?