Been so busy with deadlines and college that I plumb missed this one. Making up for it now. This is by my buddy James Reasoner (one of the writers on the Rancho Diablo series) and his wife. It's the seventh book in their long-running Wind River series.
The Wyoming Territory town of Wind River has gotten so peaceful that some people think it's downright civilized. But they don't know that a gang of vicious outlaws is planning a military-style raid on the settlement that will clean out the bank and the other businesses. Anyone who gets in their way will be cut down in a hail of bullets.