I am trying to understand how having barkskin (AC 16) and being in cover (+5 AC) = 17 AC.
If I have 16 AC because my skin turns into bark, and I dive for cover, I should have 21 AC.
Are you asking me, how I came up with my number?
Or are you thinking about any number of houserule suggestions to treat Barkskin as actual armor floating around?
Because in the first case, I can answer:
Assume a Druid with AC 12. Whether this is because he's wearing AC 12 leather, or because his Dex is 14, or because he's using a shield does not matter.
This druid then takes cover. His AC increases by +5 to AC 17.
So far so good.
Now let's repeat this sequence of events, only the Druid casts Barkskin on himself first.
His AC is still calculated to 12, but because the spell tells us his AC can't be lower than AC 16, then so be it. His AC is 16.
Now he dives for cover. Regardless of how he got his AC 12 in the first place... his AC is now 17.
Leather + Cover = 12 +5 = 17.
Dex + Cover = 10 + 2 + 5 = 17.
Shield + Cover = 10 + 2 +5 = 17.
The spell is still only telling us his minimum AC is 16. So we are unaffected by the spell, and our AC remains 17. It is not increased because it is already above the minimum. And it is certainly not decreased, since the spell only talks about a minimum, not a maximum.
Hope this makes my reasoning clear, WarHawke
(None of this changes the fact that it would probably be a lot simpler if the spell just said Barkskin acts as magical chain)