You want to shoot a brahmin out of a Fatman? That's crazy. You're crazy. Get off the road.
Sorry, a bit sleepy
You would have to create a new projectile (or edit and change the ID of an existing one) that uses the brahmin mesh as its model. Projectiles are found under Special Effects->Projectiles in the object window, and are different from "Ammunition," as you were specifying. Other than that, I don't know what you're having trouble with.
Wait... are you saying that you want to be able to shoot any given brahmin that you chance upon as the projectile? That would be beyond my knowledge...