So raiders and other generic npcs draw their weapons from a pool of items contained within a form list known as a leveled list, which produces a randomised weapon along with the corresponding ammo if any. You need to edit the form lists for raider weapons, and add your own, along with the required ammo.
The GECK guide from Bethesda actually has a full page on this you should read, but once you get your head around it, working with the levelled list is really easy. You can add new armours, guns, melee weapons... anything to spice up your game.
http://geck.bethsoft.com/index.php?title=LeveledItem
Hope it helps.
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