A repeater module is a reasonably compact, complex mechanism of wood and metal operating with sophisticated springs, that can be fitted to any masterwork light or heavy crossbow and that can hold a maximum of 20 bolts at any one time. A repeater module automatically spans the crossbow string and mounts a new bolt in place after the last one is discharged, as long as there are bolts left in the module itself. Basically, the repeater module allows you to perform a full attack action with a crossbow. When wielding a crossbow with a repeater module installed, you can make use of the Rapid Shot feat and other feats regarding ranged attacks if you have them. Loading a crossbow with a repeater module feeds the new bolts directly into a small bolt magazine imbedded in the module in a 5 by 4 arrangement. Manually reloading a repeater module, i.e. inserting the bolts in the incorporated magazine individually, takes 1d4+1 rounds, but replacing the bolt magazine itself is a much quicker process and can be done as a standard action. Non-masterwork crossbows cannot handle the strain that a repeater module puts on the mounted bow string and take 1d4 damage every time the mechanism loads a new bolt, that is not reduced by the weapon's hardness. Attaching a repeater module onto a crossbow requires 1 minute. Removing one requires half a minute.
Every round that a crossbow with a repeater module is used in combat, there is a 1 % chance of the mechanism stalling due to miscommunication between the crossbow and the module. If that happens, the crossbow cannot be used until the repeater module is removed and reattached properly. If a crossbow is magical, this chance of stalling disappears.
Construction: Fashioning a repeater module requires 500 gp worth of base materials ranging from top quality wood and steel to a carefully callibrated system of autoloading springs. Its construction requires either a DC 25 Craft(Bowmaking) check, or a DC 25 Knowledge(Architecture and Engineering) check in conjunction with a DC 20 Craft(Carpentry) or Craft(Metalwork) check.
Creating extra magazines costs 25gp worth of materials and construction requires a DC 20 Craft(Metalwork) check.
Price: 1,500 gp.
Extra Magazines: 75gp
Weight: 4 lb. (includes the weight of its bolt magazine).