At the Greenvale Sheriff's Office go downstairs to the shooting range and shoot a target until the 'Item Acquired!' message appears on screen. Then turn around and collect the item. Each target in the shooting range has a different item and you can collect as many items as you want.
When the following tasks have been completed the corresponding special weapon will become available. These weapons have unlimited ammunition and do not degrade.
Unlock .357 Magnum: Complete Sidequest #11, 'Snack For Willie' with ALL 7 bones in your inventory. Then find Forrest and Willie. Willie will take the bones. Find Forrest's truck and take the bones back.