49 C.F.R. § 178.607 – Cooperage test for bung-type wooden barrels.

(a) Number of samples. One barrel is required for each different packaging.
(b) Method of testing. Remove all hoops above the bilge of an empty barrel at least two days old.
(c) Criteria for passing the test. A packaging passes the cooperage test only if the diameter of the cross-section of the upper part of the barrel does not increase by more than 10 percent.