The game follows the same formula/structure as Swords & Wizardry Continual Light.
There are four key classes: Gunslinger, Scout, Soldier and Swindler.
There are also four optional classes: Deadeye, Moonshiner, Scrapper and Vigilante.
If you're not aware of how optional classes work in SWCL they are basically modified versions of one of the key classes. The drawback for selecting an optional class is that it takes longer to level up the character. But usually it is for good reason because some optional classes might have an advantage over the other classes.
Like I mentioned above, the game is very much like SWCL, but there are some special rules that are specific to the western theme. Now, this game does something that I love when it comes to the damage of guns: 1d6 ONLY! Now, the shotgun is 1d6+2 but it is still a single die being rolled. I love this about this game. Now, this game doesn't have armor, well except for a shield, which means there is a much higher chance of getting hit. Which is perfect for this game. The only thing I feel is missing from the combat rules are special rules when it comes to things like cover since there is no armor. For example: Receiving an AC bonus while riding a horse against an NPC who is not mounted on a horse. I mention that one as an example because in some Westerns you see the Cowboy hang from the side of his horse, using his horse as a shield, while shooting at the villains.
Included in the game are 16 Critters and Folks (The "monsters" of the system). Bandits, Outlaws, Wolves, Snakes, etc. But there is also nothing stopping you from using Zombies from SWCL.
Another great thing about Six-Shooters & Wagons (and other products from Justin Isaac) is the art! All of the artwork is Public Domain but he does a fantastic job with finding and selecting the perfect art. It has this wonderful Pulp Western feel to it. I love it!
If you're already a fan of Swords & Wizardry Continual Light or you're just looking for a Western themed game then I would highly recommend Six-Shooters & Wagons. It is rules light and basic enough to expand and modify to make it fit for you and your gaming group. So go ahead and purchase it and throw in A Fistful of Dollars while you're at it to support Justin Isaac.