Accuracy has nothing to do with chance to kill. It just determines where the missile goes. Basically the firer works out the exact angles he needs to hit the target. He then modifies these angles randomly based on the accuracy, and takes the shot with the adjusted angles.

Power has nothing to do with armour piercing ability. It does determine how many hit points of damage are done, but it's mainly there to set how much damage artillery units do against walls. You can make archers that knock down walls if you want.

There is a seperate stat that determines armour piercing ability.