If you are talking about AP rounds... all they do is increase a gun's DAM by up to 15 (only very low DAM weapons get full benefit). The effectiveness of AP ammo varies depending on the weapon you are using and the DT of the opponent. AP rounds for an extremely low DAM gun, like a BB gun, will always do more damage with AP rounds if the target has any armor at all (but there are no AP rounds for BB guns
Critical hits always ignore DT. So, a gun with a very high critical chance multiplier like a sniper rifle (5x) will almost always do more damage with any round other than AP regardless of DAM or DPS.
You could just use energy weapons and ignore the whole ammo puzzle altogether... but then you do not have the satisfaction of your gun going BOOM while the opponents head explodes like a melon.
