Black Powder Guns and Flintlocks. I think it makes sense that people would try and use that simpler older stuff before the tech level was there for more modern guns, and would make sense they kept being used as "poor people's guns".
FOnline Ashes of Phoenix has those as low-level guns and its wonderful. Dragoon Pistols, Pepperboxes, Bunderblusses, Muskets, Carbines, Nock Guns, Wall Guns... its glorious.
I think FO76 had one?
Would go REALLY hard with a Caribbean Fallout or the like.