Here’s a set of club developed American Civil War rules.
American Civil War
Gettysburg At The Gallop
From Baccus Miniatures comes Gettysburg At The Gallop, a set of free wargames rules for the American Civil War in 6mm.
Command Combat Civil War: Bull Run
Command Combat Civil War: Bull Run is both a rules set and a scenario for playing the American Civil War.
Battle for The Bliss Farm ACW Rules
Battle for the Bliss Farm is a scenario and set of free wargames rules for the American Civil War.
1862 Campaign Rules
1862 is a set of American Civil War Campaign Rules designed for Johnny Reb, but which could be used with any miniatures rules set.