GTA III 100% Walkthrough: Mission 04 - Pump-Action Pimp
Luigi is furious that a Diablo is pimping women around his turf, and tells Claude to take care of it. This mission introduces the Ammu-nation shop, where we can buy guns. Though in the mission, we are told by the shop owner to pick a Pistol at the back of the shop. Once eliminating the Diablo member and the pimp, the mission is passed and the Pistol becomes available to buy in the shop.