A cannon's cascabel comprises the knob (A) and the neck (B), with some models also having a filet (C).
By some definitions the cascabel additionally includes the base of the breech (D).
Many naval guns had a heavy metal loop on the cascabel, the pomellion, through which a wide rope was passed.
[1] Bronze cascabels from captured guns have been used to make Victoria Cross medals.
It was long thought that Russian guns captured during the Siege of Sevastopol were used, but this has been shown not to be so.