Shell Pocket is a gear perk that raises how much shotgun ammo you carry. Shotguns chew through shells in close fights, so this keeps a shotgun build going between ammo pickups.
Your reserve of shotgun shells goes up, so you run dry less often in long outpost fights.
Wear the Clown Limbs. 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.
Shotgun Rush gives a speed boost after shotgun kills, which helps you push to the next enemy.