What kind of medieval armour and weapons can you make using today's sciences?

[Aeson]

[Canister] fired much bigger rounds then grape

Pretty sure you have that backwards - grape consists of fewer, larger rounds while canister has more, smaller rounds and may use such things as fragmented scrap metal or broken nails instead.

Also, if you're looking for a sort of area-of-effect shot for cannons at long range, what you really want is something more like a shrapnel shell (spherical case shot) or a time-fused high explosive shell (impact-fused might also work, but probably won't affect as wide of an area and may bury itself in the ground prior to detonation, significantly reducing its effect; proximity-fused could be better at sufficiently long range since it would more reliably burst at the optimum height above ground for maximum effect, but is unlikely without electronics); grape and canister are more for use against nearby targets.

That's in real life, I thought the discussion was revolving around the idea that weapons like you've just described simply couldn't ever be developed in this fantasy world?

The scenario in the original post only explicitly posits a lack of electronics, and at least into the 20th Century you don't really need electronics to make explosives and propellants for gun and rocket artillery.