Plane Sailing
Astral Admin - Mwahahaha!
One thing that I've done which you might want to consider - I've swapped round the benefit for cover and concealment.
IRL, cover is better than concealment from all sources I've been able to investigate*, but in d20 world the concealment mechanic is better for you than the cover mechanic, because it gives you a chance of escaping the damage.**
So what I do is say that standard concealment gives you +4 AC and greater concealment gives you +8 AC.
For cover, the %age cover you are able to take is the %age chance that an attack hits the cover instead of you. I set the maximum cover percentage roughly as follows
Shooting over a low wall or crate: 50%
Shooting round a corner with a longarm: 60%
shooting round a corner with a sidearm: 70%
Peeping round a corner to see what is going on: 90%
and so on.
This makes taking cover a very important thing to do in a gunfight (all PCs and NPCs try to get to cover as quickly as possible when a gunfight kicks off).
The other thing that I do is to only allow defence to apply against gunfire if you either (a) moved your speed last round or (b) are adjacent to cover you could concievably duck behind.
I don't know if those ideas might prove useful to you.
* for a couple of limited examples:
http://www.officer.com/article/article.jsp?siteSection=3&id=32796
http://www.map-reading.com/ch11-4.php
** most noticeably in standard d20 if attackers are much better or much worse than you. If the attacker is much better, a +4 cover bonus to AC might not make you any harder to hit. If the attacker is so much worse that he only hits on a 20 then additional cover makes you no harder to hit because he is still automatically hitting on a 20. However, a miss chance is applied after the roll to hit - so a miss chance can still save your bacon in either of those cases (and it can even save your bacon against a critical hit - something which an AC bonus doesn't help with). That is why I maintain that miss chances are a better all round defence than AC bonuses.
Regards.
IRL, cover is better than concealment from all sources I've been able to investigate*, but in d20 world the concealment mechanic is better for you than the cover mechanic, because it gives you a chance of escaping the damage.**
So what I do is say that standard concealment gives you +4 AC and greater concealment gives you +8 AC.
For cover, the %age cover you are able to take is the %age chance that an attack hits the cover instead of you. I set the maximum cover percentage roughly as follows
Shooting over a low wall or crate: 50%
Shooting round a corner with a longarm: 60%
shooting round a corner with a sidearm: 70%
Peeping round a corner to see what is going on: 90%
and so on.
This makes taking cover a very important thing to do in a gunfight (all PCs and NPCs try to get to cover as quickly as possible when a gunfight kicks off).
The other thing that I do is to only allow defence to apply against gunfire if you either (a) moved your speed last round or (b) are adjacent to cover you could concievably duck behind.
I don't know if those ideas might prove useful to you.
* for a couple of limited examples:
http://www.officer.com/article/article.jsp?siteSection=3&id=32796
http://www.map-reading.com/ch11-4.php
** most noticeably in standard d20 if attackers are much better or much worse than you. If the attacker is much better, a +4 cover bonus to AC might not make you any harder to hit. If the attacker is so much worse that he only hits on a 20 then additional cover makes you no harder to hit because he is still automatically hitting on a 20. However, a miss chance is applied after the roll to hit - so a miss chance can still save your bacon in either of those cases (and it can even save your bacon against a critical hit - something which an AC bonus doesn't help with). That is why I maintain that miss chances are a better all round defence than AC bonuses.
Regards.