Would the anti-tank guns of WWII be effective in stopping or destroying a current day tank.

So, the German 3.7cm Pak 36 AT gun was called the "doorknocker", because except at close range, and against the vulnerable side and rear of a tank, the gun simply wouldnt penetrate. The round would make a loud BANG, alerting the tank crew to danger, and the flash would expose the guns position. Hence, "doorknocker".

Against a modern tank, the Pak36 would be completely useless. Even the big guns of the war, like the 8.8cm or the Soviet 122mm would struggle against a modern tank round.

To pull some numbers, the German 8.8 cm KwK 36, firing APCBC (Armor Piercing round, filled with explosives, designed to penetrate armor) would, at 500meters, penetrate an average of 110mm of steel armor. The Soviet 122mm m1931/37, the gun for the IS-2 and 3 tanks, using the same ammunition and at the same range, would penetrate ~125mm of armor. The Leopard 2 uses composite armor, which is made up of steel plates, kevlar pads, ceramic plates, and open space, all designed to stop an incoming round. On the L2A4, the front glacis plate has 600mm of Rolled Homogeneous Armor equivalent protection. (essentially, it would take 600mm of steel plate to equal the protection afforded the L2 by its composite glacis plate.) And, just for comparison, the Rheinmetall 120mm L55 gun, now on the L2 and the M1A2 Abrams, may have upwards of 580mm of penetration at 2000 yards (1800 meters). However, the penetration characteristics of the gun and ammunition is still classified, and is likely a lot higher. For a better comparison, the classic KwK 36 had a muzzle velocity of between 800 and 900m/s (800 for that APCBC ammo, 900 for APCR ammo, which is very similar to SABOT rounds fired today). The L55 has a muzzle velocity in excess of 1700m/s. You might notice a small difference.

Since World War Two, nearly 70 years worth of research and development has changed not only the guns involved in armored warfare, but the defenses a tank can rely on. As previously mentioned, the composite armor found in all modern tanks offers a tremendous amount of protection vs. solid steel armor. And this post did not really discuss things like Explosive Reactive Armor, which are explosive plates designed to stop, slow, or prematurely detonate incoming rounds. And too, metallurgy and materials science has developed significantly in the last 70 years. Rather than relying on pure steel to stop an attack, all sorts of metals and non-metals are now integrated into the tanks defense.

But gun-science has changed as well. As weve noted, the guns are bigger, their rounds are faster, and their better designed to penetrate a hostile tank. Yet thats only half the battle. Modern tanks have all kinds of electronic sensors which detect the guns position, the heat of the barrel, the temperature of the air, the barometric pressure, visibility to the target, and a whole host of other minor calculations which ensure that the first round hits the target. Gyroscopes keep the gun steady, even on the move, while the tank commander has access to more information than ever before. And none of this was available prior to World War Two.

Thats not to say anti-tank guns were useless, or that every World War Two gun was immediately discarded after the war. The Soviets loved the concept of an anti-tank gun, and kept the 85mm D-44 anti-tank gun around into the 60s (also, the D-44 was upgraded, in 1953, to become the D-48). And after all that, they designed a new gun, the 100mm T12 AT gun, which lasted up into the 80s.

But when you look back at it now, the technology developed in the 1940s is really outclassed by what's around today. Science and engineering have moved on, and so too has the science of killing tanks.

Correction: The M1A2 uses the RHMT 120mm L/44. The L2A6 uses the more advanced L/55.