When you jump into a PVP match your primary ammo should be well stocked, but your secondary ammo will be empty. Pistol ammo is always infinite, though. Your secondary is likely your more powerful weapon, though. Sniper rifles, rockets and more are secondaries, and ammo must be pried from slowly regenerating ammo crates around the stages. Make sure to be careful when shooting with this ammo, and use whatever gun is more plentiful if you're unsure of the accuracy.