I come pretty close:

Take the inside diameter (7') divided by 2 (3.5') and square it (12.25 sq ft.). Multiply that result by pi = 3.14. (38.48 sq. ft). Multiply this by one foot (38.48 cu ft per foot of length) and multiply this by 7.48 gal/ cu ft to get 287.86 gallons per foot of length. For 27 feet, this equals 7772 gallons. However, if the inside diameter were only 6' 6" the capacity would drop to 6702 gallons.