The aluminium biscuit has a .36" hole and the ICS Carbon biscuit has a .32" hole, so if you are going to shoot both carbon and aluminium arrows get the larger hole biscuit. A loose arrow to biscuit fit always works the best with this rest. The aluminium biscuit will work well as is for shafts up to 22xx, you can open the biscuit up by spreading the ring at the split in the biscuit to accomodate a somewhat larger shaft. A tight biscuit increases wear and tear on fletching as well as robbing speed