Question about Ammo and DR

[bwoodring]

I am trying to figure out a good solution to the AP vs. HP ammo problem in FO2, but I've run into a hole in my knowledge about the way the game calculates damage.

Is there a ceiling and floor to effective DR (modified by ammo type). For example: using AP ammo, can DR actually become negative, thus increasing the amount of damage done? similarly, is there a maximum amount of DR that a target can have, or can it go all the way to 100% if HP ammo is used?

My goal is to change the ammo types so that HP ammo is more useful against targets wearing no armor or leather armor (0-25DR), and AP is more useful against targets wearing combat and power armor (40+DR), while the two ammo types will be roughly equivalent against targets with 30-35 DR.

Really, I think AP ammo should be better starting at about 25 DR to be more realistic, but I'm afraid that would make HP ammo useless, and I'm trying to fix the problem of useless ammo.