Four Bullets by Kyle Rader

Drake and Captain Marsden rode together with a crew of morally ambiguous men for years before a deadly falling out resulted in the death of the one Marsden and Drake held dearest.

Now, Captain Marsden prepares to end his former pupil and friend once and for all. Drake is captured and brought to the town of Matherton, where five opponents await him, all of whom desire his blood by the bucket. If Drake can find and kill these assassins before they kill him, he will be allowed to leave the town alive, with a caveat:

He only has four bullets.