This is a very nifty invention.
The inventor seems to have used a magazine and bolt design similar to the ancient Chinese repeating crossbows.
From the inventors website …
The specification I devised called for a full auto-firing rate of 1 bolt per second; with 6″ of draw length and a minimum of 30lbs draw weight. The magazine was to house 10 bolts 9″ long. The bow was to be capable of discharging the full magazine with imperceptible slowing down and have an effective range of 25 yards.
The draw length is limited by the throw of the crank that pulls the bow carriage plus the relationship this has with the near stall torque that can be pulled out of the motor / gearbox assy. The need to maximise these relationships must be balanced against the voltage / number of batteries / weight.
The above was achieved using 8 AA rechargeable NiCd batteries giving 9v.