Before heading into the fray, Kenny will advise you to stock up on weapons. Locate a nearby Ammu-Nation vending machine, marked on your mini-map. Interact with it to browse available firearms. For this mission, a good pistol or SMG will be sufficient. Purchase some ammo as well. The vending machine system allows you to buy weapons and ammo at any time, provided you have the cash. It's a convenient way to re-arm yourself before difficult missions. Always ensure you have enough firepower for the task ahead, as running out of ammo in a firefight can be deadly. Consider buying some body armor if you have the funds.
