bypass armor function

Is it possible to make a weapon bypass armor? I want to make a mace that will do a consistent amount of damage across the board to any entity wearing metal armor, but I have no idea how to do something like that. Should I just make it do 0 damage and apply the amount of magic damage instead? If I do that is it possible to make the weapon's description show that amount instead of 0? I know if you set a negative damage value you're unable to hit mobs at all, is there a way to bypass that function? Sorry for the barrage of questions, I'm just very lost if you can't tell. Any help is appreciated, thank you.