The first pin goes all the way though from the outer edge of the shoulder. This will carry the weight of the weapon. Green stuff is then used to hide the hole. The 'arm' retains a degree of backwards and forwards movement. So a second pin comes in from the rear to fix it in place. This pin only goes halfway into the shoulder joint and the hole will be hidden by the torso:
The base is then shaved down ready to receive a magnet, which is greenstuffed and glued into place.
The weapon can now be rotated left and right.
Time for undercoating...