My trusted DREMEL hobby drill at work. The drill bit is 0.6 mm thick. The gun garriage is 20mm long with brass wire strips for axles. The hoops that hold the barrel is being drilled in this picture. The 0.6mm hole is drilled right through the sides of the carriages so it must be square... I had a ~95% success rate. Fortunately not all the guns on the gundeck will be fully visible so the ones where the holes exited the sides are will be part of the hidden guns.