The Collaborative International Dictionary of English v.0.48:

Rundlet \Rund"let\, n. [Dim. of OF. rondele a little tun, fr.
   rond round. See Round, and cf. Roundlet, Runlet.]
   A small barrel of no certain dimensions. It may contain from
   3 to 20 gallons, but it usually holds about 141/2 gallons.
   [Written also runlet.]
   [1913 Webster]