If you want this cheat to work you need to have down the mission 'Verdant Meadows'.

First go to the safe house in 'Tierra Robada' that you got when you unlocked the 'Toreno' Missions.

Then go inside the house and there should be four weapons, an RPG, a Heatseaking RPG, a Flamethrower, and a Minigun. if you don't know what a mini gun looks like it looks like a turrent.

Next go save the game just outside the house near the garage.

Once you've saved the game go back inside and get the minigun again, do this as many times as you wish.

If you already know how good the minigun is dont bother reading this. The minigun is probaly the best gun in the game, it can destory a car in about a second and i dont even think you reload it.