Cavalry Man: Doom Weapon by Ed Gorman

When hard-living, hard-drinking federal agent Arnold Grieves came to Junction City, he saw a town overflowing with vice—the kind of place where an unscrupulous lawman could feed his twisted hungers and make a serious killing. Now the old Grieves is gone and the new one's hell-bent on getting rich . . . by selling a doomsday weapon to evil men who are not reluctant to use it.

The powers back East suspect something—so they're sending their very best manhunter, Noah Ford, to Colorado to hunt Grieves down. Ford doesn't know it yet, but he's riding into a town that's fixing to explode with chaos, treachery, and unimaginable violence. And the rogue he's chasing's not about to surrender quietly—and neither one of them will be dying alone.