Modern ballpark equivalents of AC values

[SalterisSolaris]

Nothing overly important, but I have recently been wondering what an AC of 20, 30 or 40 would roughly correspond to in more modern terms.
And since I can't possibly be the first (and certainly not the smartest) to do so, I thought I'd ask here.

Needless to say, yes, any such comparison is probably doomed to be crooked from the start (what with magic playing into the logic), but to give an example - would an AC30 Adult Red Dragon be close to a modern tank? To a Humvee? To a normal car? What type of object/vehicle would even be a good comparison?

*scratches head*

[Florian]

AC doesn't realy represent stopping power lf armour but your ability to evade penetrating hits.
Stopping power is based on Damage Reduction.

So your tank would be AC10, DR 30/-

[hamishspence]

Modern animals with lots of "natural armor" might be a starting point.

Whales and rhinoceroses are both AC16:

http://www.d20srd.org/indexes/monsters.htm