Aquanautica Imperialis is a set of rules for naval games in the world of Warhammer 40K.
Even if you don’t like the science fiction game, Warhammer turns out to be a pretty flexible system for playing other periods. Plus, it has the advantage of familiar mechanics and a built-in player base. So given that, its not surprising that it’s been extended to practically every period you can think of. Here’s a variant of Warhmamer 40K for colonial games in darkest Africa.
Rabbidgerbal’s Super Simple 1PG Sci-fi Skermish Rules is a set of free wargames rules meant as a 40K replacement.
Yes, that’s really how the author spelled “Skirmish”
One Page 40K Rules are a set of simple rules for the 40K Universe. The authors write:
One Page 40k is a single-page ruleset designed to be fast to learn and simple to play. It serves as a great introduction to the Warhammer 40.000 universe for people who are new to the game, and as a way to play quick games for veterans of the franchise.
The Cops.NU site has a Warhammer 40K variant for the American Civil War. I think it could work.