[zxcvbob]

(the ball at the back of the breech) Is it just a decoration? Or maybe a place to wrap a chain around if you needed to pick up the barrel to lift it on/off the carriage?

[mykeal]

Cascabel is 'jingle bells' in Spanish. It's also a small, round chili pepper. And the 'knob' on the breech end of a muzzleloading cannon barrel. In that case it served essentially as a handle for moving the cannon in azimuth. Two ropes with loops in the end, one from each side, were placed over the cascabel with the loose ends in winches. This was primarily a marine application, but most barrels were built with the capability regardless of land or sea usage. The cascabel disappeared with breech loading cannons.