The weight of explosives when referred to in these rules shall not include the weight of the packing box in which the explosives are packed:
Provided that in case of explosives of the Class 6 (Ammunition) or Class 7 (Fireworks), the weight shall be deemed to be the weight of the completed article inclusive of the case or contrivance in which the explosive is contained, but shall not include the weight of the inner package and outer packing box.