Old West: Shotgun pellets and missed targets

[hal]

Hello Folks,
Suppose you have a player who is holding a shotgun in his hand such that he will aim at a target that is 15 feet away. Target gets to the hitching post and grabs the reins of the horse, and unties it. Then as he attempts to mount his horse, due to all the gunfire prior to the dramatic escape attempt, the horse is skittish and doesn't allow for an easy mount. Said person has a bulls eye on his back painted there by a PLAYER CHARACTER with a 12 guage shotgun pointed at his back. BLAMBLAM as both barrels go off. The target suffers a whopping total of 6 pellets entering his back, of which one enters his vitals. Poor guy suffers a total of 27 points of damage.

But what of the remaining 13 pellets from the shotgun blast? Since you know the pellets grouped at torso height in order to strike the fleeing man in his back, the other pellets have to be at about the same height. Assuming that if a roll is made to determine whether the remaining pellets went left of the target or right (left hits the horse, right just continues on) - how would one roll for the second grouping of 13 pellets?

Would it be treated as a roll against a skill 9 or less to hit the wrong target, and grant a bonus for rapid fire equal tothat gained by firing 13 bullets? Or would it be a simple roll against a 9 to hit and see how many pellets hit? Just curious...