Due to the fact that Jax uses alot of attackspeed it would not make any sense in my eyes to go AP [thats just me lol] since he would lose most of his high burst output to 3 maybe 2 damaging moves at the beginning. Thats mostly I went armor penetration, but dont get me wrong when I play Jax I do use around 150-200 Ap endgame. Since I still like to see glowing red bars slide and deplete [xD] I get Magic penetration Mastery since it still comes in handy when I go into battles, it helps my ulti and ap portion of my damage get through some of a tanks defences, this obviously is my first build on mobafire but if I see that you guys like it Ill continue and make this a full guide complete with screenshots :]