It is set to use "" as ammo. This is very much deliberate (and I recall making some code changes specifically to support this) because if you run out of ammo during the quest where you use it, you'd be stuck.How do you determine that the rifle does not need ammunition? One way to disable it is to change it so that it does need ammunition, preferably a type of ammunition which can only be obtained at an appropriate time if you need the rifle again in a later quest... Otherwise, if you are certain that the rifle will never be needed again, change the end of "The Cartagena Firework" so that you give it back to Augusto Queiroz.
Don't want that.