Joij21
🔥Deny, Defend, Depose!🔥
If you wanted to really simulate a primitive society(like native americans) getting guns, an auto-upgrade system for certain units could be implemented. Basically if say you have an archer unit that successfully kills a rifleman your archer would then have a percent chance to automatically upgrade into a rifleman as well(stealing the weapons like during a native raid). Or a more peaceful approach would be obtaining the rifle resource from a more advanced civ(which let's face it the AI would view you as a joke if you a stone age civ with no leverage at all demanded rifles from them, but for the sake of this I shall assume they somehow do give you the rifles) then your archer units will be able to be upgraded manually into riflemen with gold. However because your society is too primitive to have unlocked the ability to not only produce rifles but also train riflemen you will only be able to train archers and pay gold to manually upgrade them. If you want to make the guns yourself and train riflemen without wasting gold then you'll have to research the techs.