Devil Gun — J.T. Edson

It was the deadliest weapon in the West--a gun that could down a whole regiment, a whole Indian tribe of a whole town without even trying. They called it the Devil Gun.
Captain Dusty Fog of the Texas Light Cavalry had orders to capture that gun from a pair of Union Army fanatics--and out to stop him was the entire strength of the war-crazy Indian nations. The odds were not good. But Dusty had no choice--at stake were the lives of every man, woman and child in Texas.