Heavy Padding is a trade-off gear perk that greatly improves defense against armor-piercing rounds but slightly slows you down. It helps against soldiers who fire armor-piercing ammo.
Armor-piercing bullets hurt you much less, but you move a little slower.
Good if armor-piercing shooters keep downing you. Use Major Armor Piercing Protection gear first if you have it.
Rioter Helmet MK II and Rioter Pants MK II carry Major Armor Piercing Protection for a full anti-AP set.