How many US gallons does a "half butt" barrel contain?

Capacities of brewery casks were formerly measured and standardized according to a specific system of English units. The system was originally based on the ale gallon of 282 cubic inches (4.62 L; 1.22 US gal). In United Kingdom and its colonies, with the adoption of the imperial system in 1824, the units were redefined in terms of the slightly smaller imperial gallon(1.2 US gal; 4.5 L). The older units continued in use in the United States.

Historically the terms beer and ale referred to distinct brews. From the mid 15th century until 1803 in Britain "ale" casks and "beer" casks differed in the number of gallons they contained.

The tun is a cask that is double the size of a butt and is equal to six barrels and has a capacity of 216 imperial gallons (259 US gal; 980 L). Invented in Brentford, a tun was used in local breweries to measure large amounts of alcohol.

The butt of beer was equal to half a tun, two hogsheads, three barrels or 108 imperial gallons (129.7 US gal; 491.0 L). Butt is a traditional unit of liquid volume used to measure wine. In the U.S. it is defined to be equal to 126 U.S.gallons, Half would be the equivalent of 63 gallons.