Major Armor Piercing Protection is a defense gear perk that greatly reduces damage from armor-piercing rounds. It is the main counter to soldiers who fire armor-piercing ammo.
Armor-piercing bullets hit you for much less. Enemies with AP ammo become far easier to survive.
Yes, once armor-piercing shooters show up often. It is one of the best single-type defense perks.
Major Soft-Target Protection and Major Blast Protection cover the other common damage types.