When playing through the game, you eventually get the ability to take the shortcut to Cletus' gun shop. After every few missions [Say after maybe two or three], go to his store, and purchase something. On one occasion, beside Cletus there will be a gun on the table to the left of him. (Your left) Examine it, and when the game prompts you on whether to pick it up or not, say no. Cletus will then tell you he's been working on it for you, since you are a good customer. Around the time you recieve the repaired Blacktail, you will recieve the Killer 7 as a gift from Cletus.