Here are some more streamlined rules for our Weird West game.
In original D&D Armour Class started at 10 and went down to 1. Designers thinking they were clever decided it'd be easier to start than at 10 and go up instead. Why are we starting with 10? Why aren't we starting with 0 or 1? Lets do that.
Heavy Plate +6 Defence -3 Movement
Chainmail +4 Defence -2 Movement
Leathers +2 Defence -1 Movement
Shield +1 Defence
Firearms ignore Defence bonuses from anything less than Heavy Plate.
Movement penalty also applies to checks made against Desert Heat.
So far none of the players in our game have used any armour. I'm going to count the Tennessee Harrison's fringed leathers as "Leather Armour" and give him the bonus. He'll also get a benefit that lets him ignore the Desert Heat checks, based on his background.
What I expect we'll be paying a lot more attention to in our next game is the other things that provide Defence bonuses:
Additional Defence Bonuses
Cover +1 to +3 Defence
Darkness +2 Defence
Dodge +4 Defence, No attack
Evasion +2 Defence -2 Fighting
Parry +2 Defence, No attack. If opponent misses use their attack roll as your own against them.