also AP - gets -1 to damage rolls...Infantry: If a weapon's AP is Equal to or Lower than a model's armour save, no armour save is allowed (ie, a Blastmaster has AP3, so models with 3+,4+,5+,and 6+ would not get armour saves)
Vehicles: AP is irrelevant against vehicles, except that AP 1 weapons get +1 on the vehicle damage chart.
As confusing and counter-intuitive as it may be, LOWER AP is better.