Sure, use whatever subclasses/archetypes are appropriate. We had a Chinese laundrist (monk), and an Irish railroad worker (barbarian) in the party.
Once you ban casters and non-humans, switching the genre "flavor" from fantasy to western is pretty easy. There are a few stats for guns in the DMG...
I was going to ban Paladin and Ranger too, but one guy wanted to play a ranger whose background was a Civil War vet, so we re-fluffed his spells as occasional sniper abilities.