Gun and arm under construction - socket ball is made from
two clear half hemispheres available through Plastruct - main
tube of gun and arm from aluminum tube stock, centre section
of arm gray ABS tube ( Plastruct ) end section chrome plated tube
from stock. Arm section are cut and screwed to 'piston' correctly
when extended and retracted - note small screw-head half way
 along tube - body of screw acts as an internal 'stop' for the piston
 action. Claw will be added to complete the arm - constructed from
plywood as were the movie originals.