I am still the opinion that such big variations must not happen. Except you can ignore them because you customer does not mind either. But you cannot accept such variations when receiving and be accurate while shipping.
you need a fix conversion rate to manage this job.
Or you need to go for batch specific unit of measure, then each drum has to be a single batch with its own specific conversion, only then you have the relation between 1 drum and the exact net weight of the material in this drum.
if you only sell full drums, then you could store the partial filled drums in an extra storage type that is not considered during picking.
However it is considered in availability check if the stock status is the same and the storage location is not excluded from ATP.
With partially filled drums you always have a kind of trouble at a certain end in SAP, which requires extra coding to make 0.5 drums to 1 drum on a bill of lading