A Rebel In Time
From Wikipedia, the free encyclopedia
A Rebel In Time, by Harry Harrison, 1983.
A racist colonel from the modern era goes back in time to try to change the outcome of the Civil War and give victory to the Confederacy. He has a design for automatic weapons that should be constructable with 19th century technology.
A black man who's a US Army investigator goes back to try to stop him.
The book has some points of similarity to Harry Turtledove's Guns of the South. Harrison himself revisited the Civil War in his Stars and Stripes trilogy.