Outrageously exhausting (I am so unfit!) 4 mile round trip to refill my CO2 canister done!
This time I weighed the canister before and after. It weighted 15.5kg empty before the refill and 22.2kg once filled and has a capacity of 6.35kg apparently.. so, yeah.. eh.. definitely not under-filled.. now I'm curious about whether CO2 becomes heavier when it warms up..