Thanks for the feedback, even though it is quite a surprising one! I didn't expect someone would think it is tacked on, especially as the whole conception entirely revolved around dices. It started from enemies with a dice for a head, each with a number that defines their speed and would change everytime they get hit ; as dices could never be set to zero, we made them zombies.
We had several possibilities on how to attack them but the gun was the only one we kept as it was the most interesting both mecanically AND thematically : shooting dices added more holes, thus increasing their numbers and making the game more difficult over time.
The whole western atmosphere was defined afterward, as it was associated with both shooting and gambling. It was a good way to thematically limit the weapon to 6 bullets, requiring to regularly recharge and create several short phases of pressure.
Of course we could have forced the theme way more. We could have rolled a dice everytime your gun is empty in order to set the number of bullets, or have zombies start with a totally random value... but that would only have made for a worst game.