The Collaborative International Dictionary of English v.0.48:

Cascabel \Cas"ca*bel\, n. [Sp. cascabel a little bell, also (fr.
   the shape), a knob at the breech end of a cannon.]
   The projection in rear of the breech of a cannon, usually a
   knob or breeching loop connected with the gun by a neck. In
   old writers it included all in rear of the base ring.

   Note: [See Illust. of Cannon.]
         [1913 Webster]

WordNet (r) 3.0 (2006):

cascabel
    n 1: a bell attached to a sleigh, or to the harness of a horse
         that is pulling a sleigh [syn: sleigh bell, cascabel]