Way of the Lawless by Max Brand

If only the night had been dark, if the gelding had had a fair start; but the moon was bright, and in the thin mountain air it made a radiance almost as keen as day and just sufficiently treacherous to delude a horse, which had been sent unexpectedly out among rocks by a cruel pair of spurs. At the end of the first leap the gelding stumbled to his knees with a crash and snort among the stones. The shock hurled Andrew forward, but he clung with spurs and hand, and as he twisted back into the saddle the gelding rose valiantly and lurched ahead again.

Yet that double sound might have roused an army, and for the keen-eared watchers around the clearing it was more than an ample warning. There was a crash of musketry so instant and so close together that it was like a volley delivered by a line of soldiers at command. Bullets sang shrill and small around Andrew, but that first discharge had been a burst of snap-shooting, and by moonlight it takes a rare man indeed to make an accurate snapshot. The first discharge left both Andrew and the horse untouched, and for the moment the wild hope of unexpected success was raised in his heart. And he had noted one all-important fact -- the flashes, widely scattered as they were, did not extend across the exact course of his flight toward the trees. Therefore, none of the posse would have a point-blank shot at him. For those in the rear and on the sides the weaving course of the gelding, running like a deer and swerving agilely among the rocks, as if to make up for his first blunder, offered the most difficult of all targets.