Mass-effect – Mass Effect 3 Ammo Types

For the most part, nope. No reason not to always have an Ammo power activated.

An exception might be if you are specced into, for example, the final rank of Incendiary Ammo and the explosive effect would be inconvenient for some reason (Perhaps you're trying to snipe a target without blowing up a strategically placed explosive barrel or something.) Those situations should be pretty incredibly rare though.

Ammo Powers will never leave you vulnerable, they will not cost you credits, will not damage your weapons, and will not kill your fish. They will allow you to do more damage, apply useful debuffs, and, with upgrades, carry more rounds.

Like in Mass Effect 2, each individual weapon has different base damage values against Health (Red), Armor (Yellow), Shields (Blue), and Barriers (Purple).

The information linked to in the post is true for the general case - most Pistols deal the most damage to Armor. But you have variance between the individual weapons as well - the Arc Pistol is a pistol that specializes in dealing maximal damage to shields, not armor.

The exact benefit against any given defense depends on the individual weapon. (Melee and the various heavy melee abilities can be considered as weapons in this case)

That clarified, Armor, Shields and Barriers provide absolute protection until they fail (they are effectively a separate health bar), barring special abilities suck as the armor-piercing weapon mods (which allow some damage to 'penetrate' armor), and armor piercing ammo (same).