Answer
To use a gun, you need both a firearm and matching ammo; start with the Guns page before planning a long route.
To use a gun in Piggy: Intercity, you need both the firearm and matching ammo. A gun without ammo is a wasted slot.
Start with the Guns page, then check Pistol, rifle, ammo, and chest/location pages. Do not take a firearm into a long route until you know how much ammo you can carry.
Ranged weapons help control distance, but they cannot replace food, water, healing, a melee backup, or an exit route.