Somatic Weaponry( Complete Mage, p. 47)
You are adept at performing somatic spell components while your hands are occupied.
Concentration 5 Ranks, Spellcraft 5 Ranks,
When holding a weapon (or object of comparable size) in one or both hands, you can use that item to trace the somatic component of a spell, rather than using your fingers. This allows you to cast spells with somatic components even while your hands are full or occupied, as long as at least one hand is holding an item of the proper size.
This feat doesn't allow you to use somatic components while grappling, regardless of the size of your foe.
You must have a hand free to cast somatic components.