What is "grape of a cannon"

Grape \Grape\, n. [OF. grape, crape, bunch or cluster of grapes, F. grappe, akin to F. grappin grapnel, hook; fr. OHG. chrapfo hook, G. krapfen, akin to E. cramp. The sense seems to have come from the idea of clutching. Cf. Agraffe, Cramp, Grapnel, Grapple.]

1. (Bot.) A well-known edible berry growing in pendent clusters or bunches on the grapevine. The berries are smooth-skinned, have a juicy pulp, and are cultivated in great quantities for table use and for making wine and raisins.

2. (Bot.) The plant which bears this fruit; the grapevine.

3. (Man.) A mangy tumor on the leg of a horse.

4. (Mil.) Grapeshot.

   Grape borer. (Zo["o]l.) See Vine borer.

   Grape curculio (Zo["o]l.), a minute black weevil ( Craponius in[ae]qualis) which in the larval state eats the interior of grapes.

   Grape flower, or

   Grape hyacinth (Bot.), a liliaceous plant ( Muscari racemosum) with small blue globular flowers in a dense raceme.

   Grape fungus (Bot.), a fungus ( Oidium Tuckeri) on grapevines; vine mildew.

   Grape hopper (Zo["o]l.), a small yellow and red hemipterous insect, often very injurious to the leaves of the grapevine.

   Grape moth (Zo["o]l.), a small moth ( Eudemis botrana), which in the larval state eats the interior of grapes, and often binds them together with silk.

   Grape of a cannon, the cascabel or knob at the breech.

   Grape sugar. See Glucose.

   Grape worm (Zo["o]l.), the larva of the grape moth.

   Sour grapes, things which persons affect to despise because they can not possess them; -- in allusion to [AE]sop's fable of the fox and the grapes.