Thanks, guys, much appreciated!
The electronics circuit is pretty standard - a series of coils along the barrel are switched on and off in succession, which attracts and accelerates the steel BB.
Every coil is controlled by the circuit below. The control signals for each coil circuit are generated from an MCU. The coils are powered from a high voltage capacitor bank, which in turn is charged up from a battery by means of a boost DC-DC converter. The BBs are loaded by a standard servo with a disk attached to its horn. The servo first turns to 0 degrees to get the BB from the top of the clip, then turns to 180 degrees and positions the BB right in front of the barrel - hence the servo noise between shots.
circuit.jpg