This +2 repeating light crossbow is formed from densewood, and it has an unusually thick and rounded stock. It attaches to the arm of a warforged, completely covering the hand. This component only operates when attached and locked in place.
An armbow magically loads and fires with a thought, as a standard action. The weapon automatically creates bolts to keep its ammunition case filled. The case holds 5 bolts at a time; whenever one bolt is fired, another one is magically created. It can create 20 bolts per day. Thereafter, the warforged can use his own life energy to create additional bolts, at a cost of 1 hit point per bolt. At any time, the warforged can expend 3 hit points to create a bolt that is aligned as if he had cast align weapon on it. Creating a bolt is a free action. Bolts vanish if they are removed from the armbow without being shot. Hit points used to create a bolt are treated just as if the character took damage; they can be healed or repaired normally.
An armbow cannot use normal bolts (either magical or mundane); it is designed to create and fire its own magical bolts.
Moderate evocation and conjuration; CL 9th; Craft Magic Arms and Armor, align weapon, minor creation;
Price 20,000 gp; Weight 12 lb.
Source: Eberron Campaign Setting