Weight of air filled drum
How much weight will an air filled 55 US gallon (208 litre) drum support in water?
According to Archemedes' principle, the drum will support a weight equal to the weight of the volume of water it displaces. If you keep adding weight on top of the drum until it is barely floating, it will be displacing a volume of 55 gallons of water.
A US gallon (3.785 litres) of water weighs 8.33 pounds (lb).
A litre of water weights 1 kilogram (kg)
55 US gallons weighs 458 lb (208 kg). This is the weight that the drum will support (less the weight of the drum itself).