What happens to the excess damage done to cover?

example: space marine behind AP 4 cover gets shot at by single bolter round from chaos space marine. Let's say the damage is 15 points. Is the excess damage of 11 points negated and the AP protection drops to 3? Or do the 11 points go toward the space marine taking cover and the AP drops to 3?

The excess of 11 goes through and hits the marine. The AP of the cover drops by 1.

Although this example wouldn't work because the Pen of abolter would negate the cover completely and the marine would take all 15 damage, minus the Pen,which would have been used to negate the bolter's Pen.

Best to look at it as cover adding AP to whoever is behind it, and dropping by 1 if the additional cover AP is exceeded.

Example: A horde of lasgun-firing guardsmen shoot at AP 16 cover and hit the AP10 chestplate of the marine behind it. They do 23 damage. 23 - (16 + 10) = 0. But 23 > 16,so the cover AP drops to 15 and next time the marine's AP will be 15 + 10 = 25.

Bogi got it right.

Any damage in excess of the cover rating done to a target behind cover continues on to hit the original target minus the damage (or Pen) absorbed by the cover.

Rennrh said:

In your example: Chaos Marine shot does 15 Pen 4 damage

Target, behind AP 4 cover, is hit in chest (10 armour, 8 TB)

The shot's AP is first applied to the cover, which tears right through it. Thus the cover doesn't lower the damage though it does negate all of the attacks Pen. The attack deals more damage than the cover's AP so the cover is reduced to 3 AP after resolving the attack.

15 damage Pen 0 continues onto the Space Marine. His armour negates 10 damage, leaving 5 left. The Space Marine's TB is more than enough to negate this remaining damage.

(Had the cover been AP 3 to begin with the damage going into the Space Marine would have been 15 damage Pen 1, and 14 Pen 0 if the cover's AP was 5.)

Be sure to rule appropriately as a GM though. A Space Marine behind cover who shot at the Chaos Marine last turn more than likely has his head and at least one of his arms sticking out of cover. If hit there he shouldn't get cover bonus (unless he "dives for cover" an option described in the Inquisitor's handbook, differnt game but practically identical mechanics so it translates). Its still up to the GM how you want to work it though, Quick and easy for simplicity's sake, or more theatrical and dynamic.

Dive for Cover: (-10) dodge test
Success: you dodge the attack as a normal dodge AND land fully behind any cover within your AB.
Failure: You do not dodge the attack, but still land fully behind cover within your AB which is then used to mitigate the attack.
You don't have to actually "dive" it could just be a lunge etc. its really up to the GM and/or the player (if the player wants to dive let him, being prone would make following shots harder to hit).

Perfect explanations and great add on rule for dive to cover, I'll have to add that. Thanks again!

Don't think its really necissary, but I'll state it anyways:

Dive for Cover is a Dodge reaction, and as such requires, and will use up, a character's reaction for the turn.