you realize a 20mm shell wouldnt do a thing to an M1. It would make a lot of noise, but even the 20mm armor piercing ammo would just bounce off. Even MBTs and some LBTs would ignore a 20mm. a 20mm cannon is only a 70 caliber round or so, which means that it is still an anti-inf round.
now RACs5 fire around a 60mm shell(i believe the ratio is slightly larger than 10-1 as AC20 are 250mm), which, while smaller than any light tank would carry, still could do considerable damage to heavy armor. as a comparison, A10s fire a 30mm, but they use depleted uranium rounds, which are rare, hard to make, and the plane was designed around that gun, not the other way around. A 55mm sabot round would have a hard time with sloped armor, but it would do fine vs flat surfaces. the tech 1000 years in the future really isnt all that high-tech.