Weapons with a lower penatration then a armours AP cant go through it right. Or is the AP a reduction in damage, if this is so what purpose does penatration serve on a weapon

please explain to me penatration and AP in relivance to each other, Thank You

AP simply reduce the damage a weapon deals by their number, unless they're lowered by Penetration.

I'd reccommend reading the Combat section of the rulebook.

Yup, what Cifer said. The Pen is there to heighten a weapons ability to go through armour without increasing the magnitude of it's damage.

Take the Melta weapons. There's not really any personal armour that can stand up to a shot from one of those. Personal armour is utterly useless against them. These means that there's no difference between wearing Guard Flack or a quilted Vest, it will go trough either just as easily and hit the target just as hard. If there was no pen and it's damage output was simply upped by 12, then someone wearing guard flack would get hit with less then someone wearing a quilted vest even though the armours are supposed to be equally as usualness against the weapon and both would get hit with WAY more damage then they otherwise would have if the weapon had simply had a pen value.

The key thing to realise is that DH combat doesnt work like 40k combat rules. You'll be fine from there...

RoBro said:

AP reduces damage. Pen ignores that amount of AP.

A hive ganger with a Hand Cannon (Pen 2) hits a guardsman (Toughness Bonus 3) in Flak Armor (AP 4) and deals 10 points of damage. The hand cannons penetration of 2 reduces the guards armor by 2 (from 4 to 2). The remaining armor reduces the damage by 2 (from 10 to 8). The guardsman's toughness bonus further reduces the damage by 3 (from 8 to 5). So the guardsman takes 5 points of damage.