Sidearm Scavenger is a gear perk that gives you extra sidearm ammo from pickups. It helps keep a pistol-heavy loadout fed without hunting for ammo boxes.
Each time you pick up sidearm ammo, you get a bit more than normal. Over a long fight that keeps your sidearm topped up.
Wear the Munitions Bracelet in the hands slot. Gear perks are fixed to the piece of clothing that carries them, so you get this one by wearing that piece, not by unlocking it in a menu.
Sidearm Pocket on the Munitions Pants raises how much you can hold, so the two cover both ends.