If a weapon has 10 Pen and it hits something that has zero armor, does the penetration convert to damage?

No, it is lost.

Alex

Ah ok, thanks. Any idea where that is in the book?

Page 141, where it explains what the terms used in a weapon's profile mean.

Granted, it doesn't exactly say what happens with excess Pen, but since Pen is clearly different from Damage (which is listed as its own entry in the same list), and it doesn't say anywhere that Pen would be added to Damage, you are to assume that what happens is ... nothing. Because otherwise it'd say so.

Lynata has it, exact wording for you as well:

When a shot or blow from this weapon hits a target, reduce the target's Armour Points by the weapon's Penetration with results of less than 0 counting as 0 (i.e. the armour provides no protection at all)

So the negative number from High AP versus Armour is treated as 0.

The biggest plus to having "excess Pen" on a weapon is not necessarily needing to pull out a weapon with better Pen when something with heavier armor appears on the same mission.

Coincidentally, this is what keeps the humble boltgun viable later on, even when you might get, say, a melta. The three different clips allow you to switch between special ammunition as the situation demands! You can have one magazine for heavily armoured opponents, another for Hordes of mooks, etc.

Isn't direct quoting of the book a copyright issue?

In the interest of answering questions a short quote is probably not an issue. Quoting entire chapters or sections would be an issue.

Generally it is best to provide a page reference and paraphrase though, especially when dealing with core game mechanics.

As for the OP question, the above responses are correct. Pen only affects armor, it never provides additional damage through over-penetration.

Isn't direct quoting of the book a copyright issue?

Fair use.

Isn't direct quoting of the book a copyright issue?

Fair use.

Depends how much you're quoting. If it's a small snippet used to fully explain the point which helps someone understand fully then this is surely a positive thing overall for the sake of book enjoyment/understanding and just a minor point.