ACES is a pulp wild west hack for FIST.
The PDF is 8 pages, with clean, readable layout, lots of white space, and some public domain illustrations.
Lore-wise, ACES takes place in the same general timeline as FIST, just much earlier. Mechanics-wise, ACES digs deeper into the minutia of combat and tracks things like ammunition and reloading.
ACES includes a variety of new Traits, with an expected western flavor. Some of them (such as Deadeye) really expect to be interacting with a revolver, and may go rogue if exposed to more modern firearms.
ACES also includes a sample mission, which is eerily similar to the first mission of Hard West II? It also doesn't have an explicit end-point, and feels a little like it needs something to tie it together.
The supplement finishes with a mission generator, which is fun and thematic, and includes a card-drawing system for adding random encounters and complications.
Overall, if you want to play some western style FIST, this is a great place to start. It also feels like a very solid foundation to expand on. Bullet tracking opens up new possibilities for Traits and encounters. Reloading incentivizes melee combat. I think there's room for ACES to spin off into its own core system, and I'd love to see it develop further.