If the Stun Attack succeeds, depending on what the attacker is armed with, roll the following to get the Stun result:

Unarmed: 1d5-3 + SB
Knife: 1d5 + SB
Improvised Weapon (i.e. pistol grip, rifle butt, etc.): 1d10-2 + SB
Any other melee weapon: 1d10 + SB