Aquanautica Imperialis is a set of rules for naval games in the world of Warhammer 40K.
Warhammer 40K variant
Warhammer 40K Darkest Africa Variant
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
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
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.
Warhammer American Civil War Variant
The Cops.NU site has a Warhammer 40K variant for the American Civil War. I think it could work.