If there is a credible link to a US Govt definition for the capacity of a tradable barrel of oil then it should be referenced in the main article  As the article states there are several definitions for the capacity for a barrel  or  gallons  I highly doubt that the US govt is or was responsible for defining the capacity of a barrel of crude oil used for commerce or that it is a worldwide legal requirement to use a USgovt definition but if there is any such legal requirement then again a reference should be stated in the main article  To say that NYMEX has no legal right to define measurement units is a bogus argument  What NYMEX has is the right to define or specify units that are pertinent to trade  If NYMEX states that oil is to be traded in terms of barrels then it is not illegal for NYMEX to state the capacity of this thing called a barrel in terms of unambiguous STANDARD units or to point to an authoritative source authoritative as far as NYMEX is concerned that defines the capacity of a barrel