In Fable 3 for the Xbox 360, there is a free weapon pack you can download from Xbox Live that contains a rifle called the Gusket’s Musket. You can click here to login to the Xbox Live website and schedule it to download to your Xbox 360 the next time you login.
The weapon will be added to your gift queue in the Sanctuary, so check back often and it should be available when the other weapons become available during the story.