Stargrunt 40,000 is a system for converting Warhammer 40K army lists into a format usable with Stargrunt.
The Cops.NU site has a Warhammer 40K variant for the American Civil War. I think it could work.
Sgt. Perry’s Heroes offers a variant of the Legends of the Old West rules for miniature wargames set in the era of the Three Musketeers.
The Gemini rules for Warhammer 40,000 are essentially a set of house rules for Games Workshop’s flagship product.
I think that the author is correct in his premise — that the Rogue Trader rules were perhaps the best.
GrimDark Racing is a set of simple rules for races in the Warhammer 40K universe. Just use models you already have. There’s plenty of violence, of course.