Day of the Gun — William R. Cox

Logan knew he was good. But so were a lot of men who went after El Puma, the dandy killer with the fancy guns who headed the most feared gang of killers along the whole Mexican border. For as far as a good horse could run before he dropped, El Puma ruled. But even El Puma couldn’t have lasted as long as he did without the help of friends in high places on both sides of the border. Colonel Barty, with his riverboat gambler’s shirts and whiskey-red face, might be part of it. And where did Logan’s old lady love figure into all this?