Mass-effect – Do ammo powers from different squad mates stack

Yes, squad members benefit from the weapon master upgrade. Remember though that the squad bonus skill grants squad members that ammo at only 50% effectiveness. Therefore squad members ammo increases by 20%, not 40%.

The Bioware math explanation can be found here.

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).