There are several guns you are able to unlock. But first you must fully customize the first machine gun, shotgun, pistol, and magnum. (they are the weakest in the category) After you fully customized the weapons, you will be able to buy a new gun that belongs in that category.
NOTE: The Skorpion will unlock the gatling gun, but you will not be able to buy it until you have successfully beaten the game. Complete the game in under 5 hours to unlock the infinite rocket launcher. fully upgrade the s75 rifle to unlock shevas longbow.