Anzio Ironworks Monstrous 20mm
The use of 20mm precision fire started before WWII with the Finns who fielded the Lathi M39 that helped them slow and almost defeat the Soviet invasion of their borders by Stalin before he became a buddy of FDR. The Finns were able to penetrate the thin armor of pre-war Soviet tanks and force disproportionate losses on the astonished invading Russian army. They also used dummies to draw out Soviet snipers, which the 20mm Lathi effectively eliminated from a safe distance.
Beginning in 1937, the Imperial Japanese Army used the semi-automatic Kawamura Type 97 20×124mm anti-tank rifle. The Type 97 was 120 to150 pounds, had a crew of four and with no muzzle brake had horrendous recoil. It did not have any optics, just open sights so it was used mainly against light armor, from fixed defensive positions, and could fire full-auto 2,083-grain High Explosive Incendiary at 2,500 feet per second (fps) from its 7-round magazine. With the advent of the next generation of armor, both the M39 and Type 97 went the way of the horse cavalry and biplane — still used in combat, but not on the winning side.
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For an idea on just how massive the 20mm round truly is, see the following image. From left to right: 30mm, 25mm, 20mm, 50 BMG, .375 H&H Magnum, 30-06, 7.62mm NATO, 5.56mm NATO