Sniper's Pocket is a gear perk that raises your sniper rifle ammo reserve. Sniper rounds can run short on long sessions picking off guards, and this gives you more shots before you need to resupply.
You carry more sniper rounds, so you can clear more of an outpost from range before running dry.
Wear the Cyber-Pants or Marksman's Pants. 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.
Steady Aim reduces sway, and Thrifty Shooter refunds rounds on headshot kills.