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 20x124mm 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.
Building on that success, Mike Remo of Anzio Ironworks wanted to move out the limit for precision fire from 2,000 yards to a full 3,000 yards (and maybe 3,500 yards). As a result, Mike developed the Mag-Fed 20x102mm magazine fed, bolt action Ultra Long Range Sniper System to accomplish that goal. Using modern computerized optics the Mag-Fed 20mm can be effectively used for anti-materiel and counter-sniping out to well beyond .50 BMG range. Why is that significant? Enemy combatants might have access to .50 BMG or similar sniper rifles, and only a 20mm weapon can outrange and dominate that kind of threat.