Random Magic Weapons (any level)

Knockback weapons often emit a low, almost inaudible hum when drawn. Whenever a knockback weapon hits its target, it initiates a bull rush attack in addition to dealing normal damage. To resolve the bull rush attempt, treat the projectile as a Medium creature with a +8 Strength bonus. The projectile doesn't provoke an attack of opportunity, and it always tries to push the target as far back as possible. Only ranged weapons can have the knockback ability, and they bestow it on their ammunition.
Aura: Moderate evocation
Caster Level: 11th
Requirements: Craft Magic Arms and Armor, Bigby's forceful hand
Price: +3 bonus
Source: Magic of Faerûn
Arms and Equipment Guide