KE damage, as inflicted by autocannons and most tank cannons, has a simple damage formula. Suppose we’re shooting at a tank with 10 armor – to do any damage we’d need at least 10 AP.
If we have 10 AP, we’ll do 1 damage per shot. If we have less than the target’s armor, our KE gun won’t even fire. If we have more, we do an extra 0.5 damage for each point over. So against the 10 armor tank, 10 AP will do 1 damage, 11 AP will do 1.5 damage, 13 AP will do 2.5, 20AP will do 6, 28 AP will do 10 damage and oneshot the tank. The game does not display fractional health, but it does keep track of it.
The AP of a KE weapon increases by one point for every range increment it gets closer to the target, so a gun with 10AP and 2275m range will have 11AP at 2100m and 21 AP at 350m. The maximum value for KE AP is 30, you cannot go over this even with range scaling. Due to this cap and the fact that you need 18 more AP than the enemy to oneshot them, only tanks with 12 or less armor can be oneshot by KE.
Special cases are 0 and 1 armor. 1 armor takes 1 damage for every point over, aka a 1AP vs 1AV will do 1 dmg, 2AP will do 2dmg, 3AP will do 3dmg and 10AP will be oneshot. 0 armor takes double damage, so 5 KE AP is a oneshot kill.
To summarize, the KE formula is really simple: it starts at 1 damage and does 0.5 more for every AP point above the target armor.