The problem with firearms is that in Native no merchant carries them even when you enable them with the item editor. You have to change the module (see above post) to get them.
Without editing the module I know of only two ways to get firearms:
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[*]Edit the troops with the troop editor and put firearms into the bonus chests or the inventory of your player.
You have to start a new game for this.
[*]a) Change the item type of firearms and cartridges to something else and make them merchandise.
b) Load your game, wait until they appear at the merchants and buy them.
c) Save your game end exit it.
d) Change the item type of firearms and cartridges back.
e) Load your game.
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I don't know if it is exists in this version but in former versions cartridges did not reload while in the players inventory, only in the inventory of a hero.
But in my opinion these weapons are not worth the work.